普瑞斯星单孔多通道

1.5t磁共振独立射频采集通道数作为医疗影像领域的重要设备，磁共振成像技术在临床诊断中发挥着越来越重要的作用。

其中，磁共振成像的磁场强度对影像质量有着直接的影响，1.5t磁共振是临床应用中常见的一种磁场强度，也是医院中较为常见的设备。

而在1.5t磁共振设备中，独立射频采集通道数则是一个重要的参数。

1. 独立射频采集通道数的定义独立射频采集通道数是指磁共振设备中用于接收信号的通道数量。

磁共振成像是利用被检物质的信号来获取影像信息，而射频信号的接收对于成像的质量有着重要的影响。

通道数越多，设备接收到的信号越多，能够获得更丰富的数据，从而提高成像质量和分辨率。

2. 独立射频采集通道数对成像质量的影响在1.5t磁共振设备中，独立射频采集通道数的增加可以显著改善成像质量。

多通道接收可以增加信噪比，降低噪音干扰，从而获得更清晰的影像。

多通道接收还可以提高成像的速度和分辨率，减少运动伪影，对于一些需要快速成像的病例有着重要意义。

另外，多通道接收还有利于获得更丰富的功能性信息，例如脑功能成像、灌注成像等方面都能够受益于多通道接收。

3. 1.5t磁共振独立射频采集通道数的发展趋势随着医学影像技术的不断发展，磁共振成像设备也在不断更新换代，独立射频采集通道数也在不断增加。

目前，市面上已经有一些1.5t磁共振设备具备了较多的独立射频采集通道，甚至达到了64路以上。

这些设备在临床应用中表现出了更优越的成像效果，受到了广泛的认可。

4. 个人观点从目前的发展趋势来看，1.5t磁共振独立射频采集通道数的增加将是未来的趋势。

随着磁共振成像技术的不断进步，我们可以预见未来磁共振设备将会具备更多的独立射频采集通道，从而获得更加优质的影像。

作为一名医学影像工作者，我对这一发展充满期待，相信未来磁共振成像技术一定会有着更加美好的发展前景。

总结1.5t磁共振独立射频采集通道数作为影响成像质量的重要参数，在临床应用中具有重要的意义。

多通道接收能够有效提高成像质量和速度，对临床诊断具有重要意义。

1.5t磁共振独立射频采集通道数
一台1.5T磁共振成像设备通常具有16个独立射频（RF）采集通道。

这些独立射频通道可以同时接收来自患者身体的信号，并在图像重建过程中使用这些信号来获得具有更高空间分辨率和更好图像质量的成像结果。

通过使用多个独立射频通道，磁共振设备可以更好地抑制噪音干扰、减少伪像、提高信噪比，从而获得更清晰的图像。

每个独立射频通道都可以独立调节采集参数，适应不同部位和不同信号特性的成像需求。

同时，独立射频通道还可以用于某些特殊成像技术，如并行成像（parallel imaging）和磁共振波束成像（multi-channel imaging），这些技术可以加快成像速度，提高时间分辨率，并在动态成像和功能成像等方面提供更丰富的信息。

需要注意的是，不同制造商的磁共振设备可能会有不同的硬件设计和配置，因此上述通道数可能会有所不同。

此外，高场强的磁共振设备通常会配备更多的独立射频通道，以满足更高要求的成像质量和应用需求。

qsep400说明书Qsep400，Qsep400Advance高通量核酸蛋白分析系统，无需人工制胶、染色、加样、拍照，操作简便，可自动计算片段大小,结果美观易判断,可直接检测分析96孔板整板样本，最快可达1min/4样本,大幅度提高检测效率，显著降低实验成本可替代普通凝胶电泳，且结果更清晰准确。

产品特点系统类型:4通道全自动上样:可分析单个样本不浪费试剂，100个样本高通量上样，检测速度快，还可升级多板位模块，满足更高通量需求上样形式:直接兼容0.2mL离心管、8联管、12联管、96孔板等通量灵活:可单次分析单个样品，也可96个样品整板上样，无浪费抛弃式卡夹:无需制胶、灌胶，每套卡夹可分析400-1200个样品(DNA、RNA和Protein) 快速分析:可达1分钟内/4样品高灵敏度:1pg/uL高分辨率:最佳可达1-4bp(500bp以内)定量范围:DNA<50ng/uL，RNA<500ng/ul分离范围:15bp-50kb，可检测大于50kb的片段，最大可达165kb结果呈现:数字数据，凝胶成像图，毛细管电泳图软件分析:同时进行定性及定量分析，界面友好，操作人性化样品需求和消耗:最低样品需求体积1uL，上样体积小于0.1pL低成本:市场上极为经济的仪器与耗材Qsep400系列规格荧光检测连接方式:USBLED激发光源电源:100-240V便携式空气压缩机重量:26KG电压:1-15kv 尺寸:54x40x36cm传统凝胶电泳的实验流程Qsep400系列检测一步到位(PCR产物直接上样)Qsep400系列相对传统凝胶的优势全自动完成多步工作:无需配胶、倒胶、点样、跑胶、拍照;无需定时观测，安全无毒自动分析记录:无需人工计算浓度大小快速分析:1-5分钟完成原先近1小时的工作灵敏度和分辨率:1pg/uL,1-4bp(500bp以内)节约样本:仅消耗0.1L样本节约耗材用量:无需每次实验添加DNAladder,并节省大量手套、枪头和PCR管结果美观直接:多种结果呈现形式，美观直接，不会出现“边缘效应"应用小结应用领域应用实验遗传学研究蛋白分析STR/SSR微卫星分型DNA指纹图谱分析核酸蛋白互作AFLP/RFLP病原微生物检测或分型常规PCR产物检测高分辨率基因分型转基因动植物检测Genomic DNA质控高分辨率多重PCR分析食品安全分析NGS建库质控质粒DNA酶切，构造及纯度分析农渔业养殖ct/cf DNA质控合成引物（primer）分析各种疾病筛查，分子诊断总RNA、mRNA、片段化RNA、micro-RNA质控Qsep 400系列高分辨率结果1.8%琼指糖凝胶高分辨率:清楚分辨4碱基差异的238bp和242bp条带pBR322-Mspl digestQsep400系列的分辨率可高达1%。

仅供研究使用。

不可用于人或动物的治疗或诊断。

使用目的SepMate™试管用于通过密度梯度离心从人外周全血和脐带血样本中分离单个核细胞（MNCs ）。

产品描述具有内置插件的聚丙烯试管。

经伽马射线照射。

储存室温（15 - 25°C ）下存放。

使用说明确保样本、含2% 胎牛血清（FBS ）的PBS 缓冲液（PBS+2% FBS ；产品号 #07905）、密度梯度离心液（见“提示”部分）以及离心机均处于室温（15-25°C ）。

1. 使用移液器将密度梯度离心液通过SepMate™插件中央小孔小心加入SepMate™试管中。

加入体积请参考表1。

密度梯度离心液上端应淹没过插件。

注意：密度梯度离心液在使用移液器后可能会产生小气泡，这不会对使用效果产生影响。

2. 使用等体积的PBS+2% FBS 稀释样本。

轻柔混匀。

例如：使用5 mL PBS+2% FBS 稀释5 mL 样本。

3. 保持SepMate™试管垂直，用移液器沿试管壁加入稀释后的样本。

样本将会在插件上面与密度梯度离心液混合。

注意：样本可以沿管壁直接倒入。

注意不要将稀释的样本直接倒在中央小孔上。

4. 在室温下1200x g （见“提示”部分）离心10分钟，无需关闭离心机刹车。

注意：对于存放超过24小时的样本，推荐离心时间为20分钟。

5. 倒出上层液体至一个新的试管，该层含富集的MNCs 。

请勿将SepMate™试管在倒置状态下超过2秒。

注意：离心后可能会有部分红细胞（RBCs ）出现在SepMate™插件的表层，但不会对使用效果产生影响。

6. 使用PBS + 2% FBS 清洗MNCs 。

重复清洗一次。

注意：清洗细胞时建议在室温下300xg 离心8分钟，无需关闭离心机刹车。

SEPMATE™流程括号中的数字对应“使用说明”部分的步骤。

表 1：密度梯度离心液的用量 SEPMATE™ 试管 起始血液样本（mL ） 密度梯度离心液（mL ） 15 0.5 - 4.04.515 > 4 – 5 3.5 504 – 1715产品号 #15415 20个试管/包 产品号 #15420 5X20个试管/包 产品号 #15425 25X20个试管/包 产品号 #15430 50X20个试管/包SepMate™-151200 x g 下 离心 10分钟（4）加入密度梯度离心液 （1）用移液器沿试管壁加入 稀释后的样本 （3）使用PBS + 2% FBS 清洗MNCs 。

DPS-1多通道数据处理系统
刘兆瑞
【期刊名称】《分析仪器》
【年(卷),期】1993(000)004
【摘要】无
【总页数】1页(P31)
【作者】刘兆瑞
【作者单位】无
【正文语种】中文
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4.一个多通道多机实时数据处理系统 [J], 高丽萍;倪重匡;柏翔
5.杜氏藻(Dunaliella sp．)多糖DPS-1的提取分离纯化和糖基组成分析 [J], 李亚清;张华微;杨海波;王心满;崔丽华;石玉红
因版权原因，仅展示原文概要，查看原文内容请购买。

Progress in Polymer Science 37 (2012) 1657–1677Contents lists available at SciVerse ScienceDirectProgress in PolymerSciencej o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p p o l y s ciPoly(lactic acid)crystallizationSajjad Saeidlou a ,Michel A.Huneault a ,∗,Hongbo Li b ,Chul B.Park ca Department of Chemical and Biotechnological Engineering,Faculty of Engineering,Universitéde Sherbrooke,Sherbrooke,QC J1K 2R1,Canadab National Research Council of Canada,75,de Mortagne,Boucherville,QC J4B 6Y4,CanadacDepartment of Mechanical and Industrial Engineering,University of Toronto,5King’s College Road,Toronto,ON M5S 3G8,Canadaa r t i c l ei n f oArticle history:Received 25April 2012Received in revised form 13July 2012Accepted 16July 2012Available online 20 July 2012Keywords:Poly(lactic acid)Polylactide PLACrystallization Kinetics Reviewa b s t r a c tPoly(lactic acid)is a biobased and compostable thermoplastic polyester that has rapidly evolved into a competitive commodity material over the last decade.One key bottleneck in extending the use of PLA is the control of its crystallinity.Understanding the crystalliza-tion behavior is particularly crucial to control PLA’s degradation rate,thermal resistance as well as optical,mechanical and barrier properties.PLA crystallization has also been a par-ticularly rich topic from a fundamental point of view because of the existence of the two enantiomeric forms of lactic acid that can be used to control the crystallization rate but also to form high melting point stereocomplex structures.This article presents an overview of the current understanding on the fundamentals of PLA crystallization in quiescent condi-tions and on the practical means to enhance its rate.Data from the abundant literature on PLA crystallization were compiled and analyzed to provide comprehensive relationships between crystallization kinetics and the main molecular structure characteristics of PLA.In addition,the most promising efforts in enhancing PLA crystallization kinetics through plasticization or heterogeneous nucleation were reviewed.© 2012 Elsevier Ltd. All rights reserved.Contents 1.Introduction ........................................................................................................................16582.Chain structure .....................................................................................................................16583.Crystal structure ....................................................................................................................16594.Structure–properties relationship ..................................................................................................16594.1.Glass transition temperature ...............................................................................................16594.2.Melting temperature and equilibrium melting point ......................................................................16604.3.Maximum achievable crystallinity..........................................................................................16625.Crystallization kinetics .............................................................................................................16625.1.Kinetics through visual observation ........................................................................................16625.2.Kinetics through calorimetry ...............................................................................................16656.PLA heterogeneous nucleation and plasticization .................................................................................16666.1.Nucleation . (1666)6.1.1.Chemical nucleating agents .......................................................................................16666.1.2.Mineral nucleating agents anic nucleants . (1667)∗Corresponding author.Tel.:+18198218000x65580;fax:+18198217955.E-mail address:michel.huneault@usherbrooke.ca (M.A.Huneault).0079-6700/$–see front matter © 2012 Elsevier Ltd. All rights reserved./10.1016/j.progpolymsci.2012.07.0051658S.Saeidlou et al./Progress in Polymer Science37 (2012) 1657–16776.1.4.Bio-based nucleants (1667)6.1.5.Carbon nanotubes(CNT) (1668)6.1.6.PLA stereocomplex (1668)6.1.7.Nucleation based on inorganic–organic hybrids (1669)6.2.Plasticization (1670)bination of nucleation and plasticization (1672)7.Conclusions (1673)Acknowledgement (1673)References (1673)1.IntroductionPoly(lactic acid)or PLA is a biodegradable polymer that can be produced from annually renewable resources[1]. It is an aliphatic thermoplastic polyester that boasts a high modulus,high strength and good clarity.Therefore, it has raised a lot of interest as a potential replacement for petroleum-based polymers.Before its introduction as a packaging and commodity material,specialty grades of PLA had been developed for biomedical uses.Its biocompatibil-ity and bioresorbability had made it a suitable choice for applications such as drug delivery systems,sutures,blood vessels,etc.The commercial introduction of bio-based PLA in2003has opened the way for more common applications. In particular,PLA has beenfinding an increasing number of applications in the packaging industry due to its good mechanical properties,transparency and compostability.The term poly(lactic acid)is slightly misleading.The PLA now commercialized for commodity applications is made from ring-opening polymerization of lactide,a dimer of lac-tic acid.Therefore,from a nomenclature point of view,we should refer to polylactide rather than to poly(lactic acid) but both terms are used indifferently in the scientific lit-erature.Another precision that needs to be made is that PLA does not refer to a single material but rather to a fam-ily of materials with a range of properties due to the chiral nature of lactic acid as we will explain later.One general drawback of the PLA family of material is that they exhibit a lower glass transition temperature(T g),up to about 60◦C,compared to competing polyesters.The ubiquitous polyethylene terephthalate,for example,possesses a T g around80◦C.Therefore,unless PLA can be crystallized to a large extent,its thermal resistance will remain relatively poor.For example,heat deflection temperature(HDT)and Vicat penetration temperature were increased more than 30and100◦C respectively,after amorphous samples were fully crystallized.As well,an increase inflexural modu-lus and strength by25%and increased impact resistance were reported following the full crystallization of amor-phous PLA[2,3].On the other hand,if one is interested in maintaining the PLA clarity or maximizing the biodegrad-ability of PLA,it might be useful to understand how to limit crystallization.Enzymatic degradation rate can be reduced by more than7times for highly crystalline PLA compared to the amorphous samples[4].Furthermore,barrier prop-erties are improved due to PLA crystallization.A study by Drieskens et al.[5]showed for crystallized PLA that oxygen and water vapor permeability coefficients were decreased by more than4and3times respectively,compared to amorphous references.This stresses the importance of PLA crystallization not only from a fundamental point of view but also for obvious market development considerations. Several authors have reviewed the synthesis,properties, processing and applications of PLA[1,6–10].The current review will focus specifically on the current understanding of PLA crystallization.In particular,we will examine the microstructure of PLA,the isothermal and non-isothermal crystallization kinetics and will summarize the different strategies used to control or enhance crystallinity devel-opment during melt processing operations.2.Chain structureTo understand the crystallization behavior of PLA,it is useful tofirst examine its chain structure.PLA can be syn-thesized by two polymerization routes,polycondensation of lactic acid or ring-opening polymerization of lactide[8]. In both cases,lactic acid is the feedstock for PLA produc-tion.Due to an asymmetric carbon atom,lactic acid has two optically active forms called l-lactic acid and d-lactic acid.When producing PLA from lactide,three forms are possible:the ll-lactide made from two l-lactates,the dd-lactide from two d-lactates,and the ld or meso-lactide made from a combination of one l-and one d-lactate.In Fig.1,schematics of the lactic acid and lactide molecules are illustrated.The polymers coming from pure l-or pure d-feed are referred to as PLLA and PDLA mercial PLA grades however are usually based on an l-rich mixture as the majority of bacteria used in fermentation processes produce l-lactic acid predominantly.Due to purification issues,they typically comprise a minimum of1–2%D units. Since the two repeating units are optically active,they rotate polarized light in opposite directions.Specific opti-cal rotation values in chloroform at25◦C([˛]25)equal to −156◦and+156◦are commonly used in the literature for 100%pure PLLA and PDLA,respectively[11–15].A higher content of one repeating unit in polymer chain results in a higher rotation angle in that direction.Thus,by employing the following equation[16],one can calculate the molar fraction of D units(X D)in PLA:X D=[˛]25+156312(1)Based on the molar fraction and source of D units in PLA chains,i.e.,dd-lactide or meso-lactide,another importantS.Saeidlou et al./Progress in Polymer Science37 (2012) 1657–16771659Fig.1.Stereochemistry of lactic acid and lactide molecules.parameter called the average isotactic sequence length(¯ ) is defined for l-lactide rich PLA as:¯ =aD(2)where a is a coefficient that depends on the source of D units in polymerization feed.It is equal to1if all D units are incorporated via meso-lactide,equal to2if they are all comprised of dd-lactide and between1and2depending on the ratio of meso and dd-lactide in the polymerization feed. Using the coefficient2for dd-lactide is due to the paring of D units in the random copolymerization of ll-lactide and dd-lactide.Another clarification about the latter equation is that it is correct for random copolymers.In some cases by employing specific initiators it is possible to have a prefer-ential monomer insertion into the growing chain resulting in a PLA with longer isotactic sequences compared to a statistical copolymer with the same X D[13,14].A higher¯ value means a higher level of chain order. Therefore,this parameter influences directly the crystal-lization behavior of PLA.It can be controlled by adjusting the ratio of ll,dd and meso-lactide in the monomer feed for PLA polymerization.However,in the course of polymeriza-tion,L or D units may convert into their counterpart form [17].This undesirable reaction called racemization will influence¯ and thus will disturb chain order.Another way that chain order may be disturbed is by inter or intramolec-ular trans-esterification reactions[11].Chain architecture is another aspect of chain structure. PLA is typically linear in its structure,but it is possible to produce it in different branched architectures by employ-ing multifunctional initiators[18–20]or co-monomers bearing initiation groups[21,22]in polymerization reac-tion.Multifunctional chain extenders[23,24]or peroxides [25,26]sometimes used for PLA to counterbalance chain scission are other potential sources of branching.Accord-ingly,it is useful to understand the effect of branching parameters such as branch’s length,amount and architec-ture on PLA crystallization behavior.3.Crystal structureDifferent crystal structures have been reported for PLA, the formation of which depends on the crystallization conditions.The most common␣-form occurring in con-ventional melt and solution crystallization conditions was first reported by De Santis and Kovacs[27]and investi-gated further in a number of studies[28–30].Based on WAXD and IR data,Zhang et al.reported the slightly dif-ferent␣ -form for PLA crystallized below120◦C[31].The chain conformation and crystal system of the␣ -form is similar to␣structure,but with a looser and less ordered chain packing.More recent studies suggest that only the ␣ crystal is formed at crystallization temperatures below 100◦C while crystallization between100and120◦C gives rise to the coexistence of␣ and␣crystal structures[32,33]. As a consequence of its looser chain packing and disor-dered structure,the␣ crystal leads to a lower modulus and barrier properties and to higher elongation at break compared to␣crystal[34].A␤-form,first observed by Eling et al.[35],is created by stretching the␣-form at high draw-ratio and high temperature such as in hot-drawing of melt or solution spunfibers[29,35].Melting tempera-ture of the␤structure is about10◦C lower compared to the␣crystal,implying that␤form is thermally less sta-ble[29].Later,Puiggali et al.[36]suggested that␤-form crystal is a frustrated structure with a trigonal cell con-taining three chains which are randomly oriented up and down.A more ordered crystal modification called␥was also reported by the same group[37].In the␥-form which was obtained by epitaxial crystallization of PLA on hex-amethylbenzene,two chains are oriented antiparallel in the crystal cell.Besides the homo-crystallization of PLLA and PDLA,these two enantiomeric chains can co-crystallize together and form a stereocomplex[38].In contrast to PLLA or PDLA homocrystals,the stereocomplex crystal cell con-tains one PLLA and one PDLA chain.Interestingly,melting point of the stereocomplex is about50◦C higher than that of PLA homocrystal.Thus,stereocomplexation may provide greater temperature resistance to the material.Properties of PLA crystal form are summarized in Table1.The densi-ties were calculated based on the reported cell parameters and the number of monomers in each unit cell.4.Structure–properties relationship4.1.Glass transition temperatureThe glass transition temperature,T g,plays an important role on the determination of PLA crystallization window since polymer chain mobility is related to T−T g.Fig.2 presents T g data as a function of molecular weight for dif-ferent d-lactate contents.The T g increases rapidly when the molecular weight is increased to80–100kg/mol but then reaches a constant value.At a given molecular weight,an increase in optical impurity,i.e.increase in minor unit con-centration(defined as d-lactate in the case of an l-rich PLA and as l-lactate for a d-rich PLA),decreases the glass tran-sition temperature to some extent but its effect on T g is not as significant as on T m.1660S.Saeidlou et al./Progress in Polymer Science37 (2012) 1657–1677Table1Properties of different PLA crystal types.Crystal type Crystal system Chain conformation Cell parameters theoretical(g/cm3)a(nm)b(nm)c(nm)˛(◦)ˇ(◦) (◦)␣[27]Pseudo-orthorombic103helical 1.070.645 2.78909090 1.247␣[30]Orthorombic103helical 1.050.61 2.88909090 1.297␤[29]Orthorombic31helical 1.031 1.8210.90909090 1.275␤[36]Trigonal31helical 1.052 1.0520.889090120 1.277␥[37]Orthorombic31helical0.9950.6250.88909090 1.312SC[39]Triclinic31helical0.9160.9160.870109.2109.2109.8 1.274SC[40]Trigonal31helical 1.498 1.4980.8790901201.274Fig.2.T g vs.M n for different d-lactate concentrations.Data adapted from[41–44].The data points for different d-lactate contents pre-sented in Fig.2have beenfitted with the predictions of the Flory–Fox equation given byT g=T∞g−KM n(3)where T∞g is the glass transition temperature for infinite molecular weight,K is a constant and M n is the number-average molecular weight.Accordingly,we have found that K increases linearly with d-lactate concentration and that T∞g shows a decreasing trend that can be predicted quite well with a rational function.T∞g=13.36+1371.68X D0.22+24.3X D+0.42X2D(4) K=52.23+791X D(5)where T∞g and K are respectively expressed in◦C and in ◦C kg/mol.The solid curves in Fig.2are the relationships obtained using Eq.(3)and the equation parameters T∞g and K described above,showing that T g of PLA can be correctly estimated from these equations.PLA chain architecture is another influential parameter for T pared to the linear structure,a branched PLA has a lower T g value.This is due to the existence of a higher free volume caused by the higher number of chain ends. Pitet et al.[21]reported a10◦C decrease in T g for hyper branched PLA produced by copolymerization of lactide and glycidol while Zhao et al.[45]reported a5◦C decrease in TgFig.3.Melting temperature as a function of d-lactate content. Data adapted from[44,47–49].for a32-arms star shaped PLA produced by a poly(amido amine)dendrimer initiator.For lower branching contents such as star or comb like architectures with4–9arms[20] or by adding chain extender[46],no significant difference in T g was reported.4.2.Melting temperature and equilibrium melting pointFig.3compares the melting point data from several authors as a function of D-unit content in the polymer structure.Pure PLLA(0%data)exhibits the maximum melt-ing temperature,between175and180◦C depending on authors.The melting point decreases linearly with the d-lactate content.The best linearfit for each data set is presented as well.The slope of these lines varies between −5.5and−5.0,meaning that1%D-unit content results in approximately5◦C reduction in melting temperature.The difference between data presented in Fig.3is due to the dif-ferent molecular weight.For example,Kolstad reported T m for PLA with M n between50and130kg/mol[47]while data reported by Witzke[44]and Bigg[48]concern PLA with M w higher than100kg/mol.Furthermore,the thermal history applied by the authors differed slightly.The melting temperature of a polymer is expected to increase with the temperature at which it was crystallized (T c).Due to the kinetic barrier for crystallization when approaching the melting temperature,polymer crys-tallization is typically carried out at high undercoolings which limits the crystal thickness.However,in the limitingS.Saeidlou et al./Progress in Polymer Science37 (2012) 1657–16771661Fig.4.T m and T0m as a function of minor repeating unit concentration,filled symbols for PDLA and open symbols for PLLA.Data adapted from[16].case when crystallization is in equilibrium with melting of crystals,the crystal grows large enough in all directions so that the melting point reaches its maximum value,the so-called equilibrium melting point(T0m).The effect of the D-content on the T0m of PLA can be calculated based on Hoffman–Weeks procedure[50].Fig.4addresses the effect of crystallization temperature and of minor unit concentration on melting point and presents the T0m values obtained for each minor unit concentration(i.e.the T c=T m line).As expected,T m decreases with minor unit concen-tration for all crystallization temperature.The melting point for any given minor unit content clearly increases as a function of the crystallization temperature.The data at T c=T m is compared to the Baur Model prediction.This model is used to calculate melting point depression due to random copolymerization with a non-crystallizable co-monomer.The Baur model[51]is a modification based on earlier development proposed by Flory[52]and is given by1T0m(copolymer)−1T0m(homopolymer)=−R(ln X−(1/¯ ))H0m(6)where X is the molar fraction of crystallizable units, H0m is the equilibrium enthalpy of fusion,R is the gas constant and¯ is the average sequence length of crystallizable units (see Eq.(2)).The model is identical to the one proposed by Flory except for the1/¯ term.The equilibrium melting point depression is well described by the Baur Model. The validity of the Baur equation was also reported by Huang et al.[53]as well as Baratian et al.[54]for PLA systems.The reported equilibrium melting temperature and equilibrium enthalpy of fusion of PLA are summarized in Table2with their calculation methods.Most authors report equilibrium melting temperatures between200and 215◦C.Variations may be due to variations in molecular weights and purity of the investigated polymers.In termsofFig.5.PLA Melting point as a function of molecular weight.Data adapted from[16,41,43,44,54,63,64].equilibrium melting enthalpies,estimations vary between 80and135J/g.Molecular weight is another factor that significantly influences the melting temperature.Fig.5illustrates the melting point variation with the number averaged molecular weight M n.The data has been compiled from seven papers where the PLA had less than1.25%minor units [16,41,43,44,54,63,64].The melting temperature increases dramatically with molecular weight for low M n but reaches an asymptotical value at M n>100kg/mol.It can be as low as 90◦C for PLA oligomers and increases up to185◦C for PLA in the100kg/mol range.It is noteworthy that commercial PLA grades with a molecular weight in the50–150kg/mol range are in the high-molecular weight plateau region and there-fore are not highly sensitive to molecular weight changes. In order to express the relationship between melting point and molecular weight in a simple way,the data from Fig.5 wasfitted using the following equation:T m=T∞m−AM n(7) with T∞m=181.3◦C and A=1.02×105◦C g/mol.This is a common molecular weight dependency used for expressing changes in polymer properties such glass tran-sition temperature,tensile strength,etc.The property is expressed as a function of the number-averaged molecular weight M n,and of a theoretical property value obtained at infinite molecular weight.As can be seen on thefigure,Eq.(7)gives a fair account of the effect of molecular weight on the melting temperature.In some circumstances,two peculiarities are observed in the DSC heating scans of semi-crystalline PLA.One is the emergence of a small exothermic peak just before the melting peak and the other one is the occurrence of a double melting peak.These two phenomena can be well explained by taking into consideration the crystallization conditions in parallel with the␣ and␣crystal formation requirements[32,33,65,66].When PLA is crystallized at temperatures corresponding to␣ crystal formation,the small exotherm appearing just before the single melting peak is due to the transformation of disordered␣ crystals1662S.Saeidlou et al./Progress in Polymer Science 37 (2012) 1657–1677Table 2Reported equilibrium melting temperature and melting enthalpy for PLA.T 0m (◦C)H 0m (J/g)MethodValueRef.MethodValueRef.Hoffman-WeeksBaur200199[16]Baur82[16]Gibbs–Thomson Data fitting214215[53]–100[53]Hoffman-Weeks 207[55]Extrapolation to infinite crystal thicknessFlory model for solution grown crystals8193[56]Hoffman-Weeks 205[57]Hoffman-Weeks 206[58]Hoffman-Weeks 211,212[59]Hoffman-Weeks215[60]Hoffman-Weeks 215±10[28]Extrapolation to crystal density (density-enthalpy relation)135[61]MarandHoffman-Weeks 227199[41]Hoffman-Weeks (pseudo-equilibriumlamellar crystals)215[62]to the ordered ␣-form.On the other hand,a double melting behavior appears when the crystallization temperature is situated in the region of simultaneous ␣ and ␣type forma-tion.For high crystallization temperatures,only ␣crystals are produced leading to a single melting peak.Effect of branched structure on the melting point was also reported in a number of studies [20,23,45,46,67].T m was insensitive to branching when different contents of reactive copolymers (chain extenders)were employed to produce long chain branched PLA [23,46].However,in the case of star-shaped PLAs synthesized by multifunctional initiators,the magnitude of T m reduction was in direct relation with the number of arms.T m reduction between 5and 40◦C were observed for branched PLAs with 4–9arms [20,67]and 32arms [45],respectively.This behav-ior was attributed to the poor folding property of branched architecture due to steric hindrance as well as crystal imperfections caused by chain ends and branching points.4.3.Maximum achievable crystallinityBoth the molecular weight and d -Lactate content deter-mine the maximum achievable crystallinity.The enthalpy of fusion data obtained from various data sets are summa-rized in Fig.6.The maximum enthalpy of fusion decreases generally with molecular weight and minor unit concen-tration.The molecular weight effect can be explained by the higher restrictions of chain motion at higher molecular weights,while the reduction in maximum achievable crys-tallinity by increasing d -lactate content is expected from crystal disruption.At about 10–12mol.%(in the case of ran-dom distribution)of non-crystallizable unit,crystallinity is extremely low and so lengthy that PLA can be consid-ered completely amorphous.Furthermore,crystallinity is diminished if branching is imparted to PLA structure as a consequence of more difficult chain segment transporta-tion to crystallization pared to linear PLA,7–15%less crystallinity was achieved for star-shaped and long chain branched PLA [20,23,45,67].Fig.6.Effect of molecular weight and minor unit concentration on max-imum enthalpy of fusion.Data adapted from [16,47,53,63,68–73].5.Crystallization kinetics5.1.Kinetics through visual observationThe overall crystallization kinetics is typically examined in terms of two independent phenomena:initial crystal nucleation and of subsequent crystal growth.In practice,optical microscopy on thin polymer films is used to deter-mine the nucleation density and spherulite growth rates in isothermal conditions.The polymer film is usually first melted and rapidly cooled to the desired temperature.The size and number of spherulites can then be monitored over time.High-quality images and more accurate mea-surements are also reported by observation via an atomic force microscopy technique [74,75].The relation between the number of crystallization sites (spherulite density)with crystallization temperature for PLA is illustrated in Fig.7.The spherulite density was shown to decrease with tem-perature and the decreasing rate gradually accelerates with temperature.The growth phenomenon is evaluated by measuring spherulite radius with time.The crystal growth rate (G )S.Saeidlou et al./Progress in Polymer Science37 (2012) 1657–16771663Fig.7.Spherulite density as a function of crystallization temperature, Data adapted from[16,72,76,77].is equal to the slope of the spherulite radius vs.time curve,while extrapolation of this data to zero-radius can be used to determine the induction time(related to nucleation kinetics).Usually G is constant for a specific T c,implying a constant concentration of impurities like non-crystallizable segments at the growth front because of their rejection to inter-lamellar regions.One of the most important theories on polymer crystallization is the Hoffman–Lauritzen theory that deals with the crys-tal growth kinetics[78,79].It defines three crystallization regimes based on the ratio between the rate of surface nucleation and the rate of chain deposition on the crys-tal surface.It is noteworthy that this theory concerns the secondary nucleation occurring on preformed lamellae.It is different from the primary nucleation defined as the ini-tiation of a new lamella from a polymer melt.In regime I which covers low undercoolings,surface nucleation is slow and is the limiting factor while chain mobility is high.By decreasing temperature and moving to regime II,surface nucleation becomes more effective while chain movement is reduced.However the combination of the two factors gives higher growth rates.Finally,upon further cooling,we move to regime III.Contrary to regime I and due to high undercoolings,surface nucleation is maximum and chain motion is the limiting factor,resulting in lower growth rates compared to regime II.According to this theory,the crystal growth rate(G)of a homopolymer is given by:G=G0e−U∗/(R(T c−T∞))e−K g/(T c Tf)(8) where G0is a pre-exponent constant known as front factor, U*is the activation energy for local motion,R is the gas constant,T c is the crystallization temperature,T∞is the temperature at whichflow ceases, T is the undercooling (T0m−T c),and f is a factor to account the change in heat of fusion with temperature.K g known as the nucleation constant is a parameter given by:K g=ab e T0mk H f(9)where a is a constant that depends on the crystallization regime(2for regime II crystallization and4for regime I and III),b is the surface nucleus thickness, is the lateral sur-face free energy, e is the fold surface free energy,T0m is the Table3Hoffman–Lauritzen Eq.parameters for PLLA.Parameter Description ValueU*Activation energy forlocal motion6.27×103J/mol[55]R Gas constant8.314J K−1mol−1T∞Temperature at whichflow ceasesT g–30KT Undercooling T0m−T cf Factor to account thechange in heat of fusionwith temperature2T c(T0m+T c)T0m Equilibrium meltingtemperature(see Table2)b Surface nucleusthickness5.17×10−10m[28]Lateral surface freeenergy12.03×10−3J m−2[55]e Fold surface freeenergy60.89×10−3J m−2[55]H f Heat of fusion(see Table2)k Boltzmann constant 1.38×10−23J K−1equilibrium melting temperature, H f is the heat of fusion for100%crystallinity and k is the Boltzmann constant. Typical values of Hoffman–Lauritzen equation parameters reported for PLLA are summarized in Table3.When the growth rate measurement is done for dif-ferent crystallization temperatures,plotting ln(G)+U∗/(R(T c−T∞))vs.1/(T c Tf)will lead to a linear plot where the slope is−K g and the intercept is ln(G0).In addi-tion,the regime change temperature can be obtained from the points of slope variation.Regime I–II tran-sition for PLA was reported to occur at163[55]or 147◦C[41]while regime II–III transition occurs at120◦C [32,41,80,81].Furthermore,G is maximum at about130◦C [32,53,55,80,82].In some cases,however,an unusual behavior is reported for the variation of G with T c for PLA where two local maxima are observed at temperatures around105–115◦C and125–135◦C instead of a bell-shaped curve[41,62,75,76,81,83–86].The origin of this double peak behavior is not known exactly.Transition from regime II to III[41,81,87]or growth of␣ and␣crystal structures [84]are assumed to be the main causes for such behavior. Interestingly,the temperatures for regime II–III transition and transition from pure␣to a mixture of␣ and␣crystal formation coincide(120◦C).Besides,there are arguments on whether the crystal structure or spherulite morphology remains the same[41,83,84]or varies for these two growth rate peaks[76].In Table4,some of the reported values for K g and G0are summarized.Reported G0data are varying widely,how-ever K g values are more consistent.Additionally,ratios of K g(III)/K g(II)and K g(I)/K g(II)are close to the theoretical value of2.For high melting point polyesters,like PLA,Hoffman et al.[88]related the lateral surface free energy to chain flexibility through the following equation:∼= H fa021C∞(10) where a0is the surface nucleus height and equals to5.97˚A for PLA according to Kalb and Pennings[28].C∞is the。

科瑞达生物微流控化学发光法
科瑞达生物是一家提供快速可靠POCT解决方案的医疗诊断公司，在微流控和化学发光领域拥有数十项专利技术。

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5t人体磁共振多通道射频激发方法磁共振成像是一种常用的临床医学检查方法，它通过利用磁共振现象来观察人体的内部结构，并提供高质量的图像。

而射频激发是磁共振成像中的一个重要步骤，它通过发送特定的射频脉冲来激发人体组织中的核磁共振，从而实现对其进行成像。

传统的射频激发方法主要采用单通道方式，即通过一个发射线圈来发送射频脉冲。

然而，单通道方式在以下几个方面存在一些限制：首先，由于射频线圈的设计和制造问题，单通道方式只能针对特定部位的射频激发，无法同时激发多个部位。

其次，单通道方式的射频脉冲无法根据不同组织的特性进行动态调整，从而影响了成像质量。

此外，单通道方式在特定部位的成像时间较长，无法满足快速成像的需求。

为了克服这些限制，研究人员提出了多通道射频激发方法，即通过使用多个独立的发射线圈来实现对不同部位的射频激发。

多通道射频激发方法主要包括并行传输技术和多元射频激发技术。

在并行传输技术中，每个发射线圈都独立工作，可以分别发送特定的射频脉冲。

这种方法可以同时激发多个部位，并且可以实现动态的脉冲控制，从而提高成像速度和质量。

并行传输技术需要使用独立的发射线圈，并且需要复杂的硬件设备来实现数据的并行传输和接收。

多元射频激发技术是另一种多通道射频激发方法，它通过使用一个多通道发射线圈来发送复杂的射频脉冲。

多元射频激发技术可以根据不同组织的特性来调整射频脉冲的参数，从而实现对不同组织的有效激发。

该方法可以减少激发脉冲的数量，降低成像时间，并提高成像质量。

多元射频激发技术需要设计和制造复杂的多通道发射线圈，并且需要精确的脉冲控制和数据处理算法。

无论是并行传输技术还是多元射频激发技术，多通道射频激发方法都需要有可靠的硬件设备和有效的数据处理算法来实现。

近年来，随着磁共振成像技术的不断发展，多通道射频激发方法已经得到了广泛应用。

它在提高磁共振成像速度、降低辐射剂量和改善成像质量等方面具有重要意义。

综上所述，5T人体磁共振多通道射频激发方法是一种有效的磁共振成像技术，它可以通过使用多个独立的发射线圈来实现对不同部位的射频激发。

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