星形聚苯乙烯的玻璃化转变温度和液1-液2松弛

用扭辫仪(TBA)和线膨胀仪测定了一组规则的星形聚苯乙烯的玻璃化转变温度T_g和T_ll松弛。两种实验方法的结果表明:支链分子量相同的星形结构聚合物的T_g随着支化度增加而升高,但也发现星形聚苯乙烯的T_g却低于具有相同分子量的线形聚苯乙烯的T_g·液₁-液₂松弛温度T_ll和损耗峰的强度也依赖其支化度。与以二乙烯基苯为凝胶核的星形结构相比较,证实了后者的双T_g转变。     

Examination of Polystyrene by Inverse GC: Part 1. Below Glass Transition Temperature

The surface properties of polystyrene were investigated by inverse gas chromatography at infinite dilution region. The standard enthalpy (??H ⁰), entropy (??S ⁰) and free energy (??G ⁰) of adsorption were calculated for the adsorption of n-alkanes (n-hexane, n-heptane, n-octane, n-nonane, n-decane) and some polar probes (acetone, ethyl acetate, DCM, chloroform, THF) on polystyrene. Additionally, the dispersive (London) component of the surface free energy (?? _S ^d ),  surface Lewis acidity (K _A ) and Lewis basicity (K _D ) of polystyrene surface were determined. The values obtained for K _A and K _D by three different methods demonstrate that polystyrene surface has a basic nature.     

1,2-丙二醇水溶液玻璃化转变与结构松弛

为了考察水含量对1, 2-丙二醇水溶液玻璃化转变和结构松弛参数的影响, 用差示扫描量热法(DSC), 测量了5种高浓度1, 2-丙二醇水溶液(60%、70%、80%、90%、100%, w)玻璃化转变区域的表观比热容. 用5种降温速率(1、2、5、10、20 K·min-1)和10 K·min-1的升温速率获得玻璃化转变的相关参数. 玻璃化转变温度分析结果表明, 虽然水含量增加能从总体上降低体系的玻璃化转变温度, 但与纯羟基类多元醇相比, 水对1, 2-丙二醇的增塑作用并不显著. 结构松弛活化能计算结果表明, 体系水含量的增加能明显降低结构松弛活化能. 脆度分析结果表明, 随着体系水含量增加, 动力学脆度逐渐降低, 但热力学脆度是先升高后降低, 在80%左右达到最大值. 结构松弛协同重排域计算结果表明, 当浓度由60%增加至100%时, 玻璃化转变特征长度由2.79 nm增加至3.57 nm.     

Examination of Polystyrene by Inverse GC: Part 2. Above Glass Transition Temperature

In this study, adsorption of some hydrocarbons on polystyrene was investigated with inverse gas chromatography. An experimental study was carried out above glass transition temperature. Thermodynamic parameters belonging to the polymer-solute system, weight fraction activity coefficients, Flory?CHuggins interaction parameter and solubility parameter for solute and polymer were calculated from the obtained data. The results were discussed with respect to the changes in hydrocarbon??s carbon number and temperature.     

An Electric Criterion to Evaluate Glass Transition Temperature: Dielectric Relaxation Measurements

In this contribution, a dielectric measurement technique for the evaluation of phase transition temperature and the study of physical aging on polymeric thin films is considered. This kind of measurement provides the possibility of displaying phase transitions with a high degree of precision. Furthermore, it can be considered alternatively to techniques not applicable in the case of thin films, such as Differential Scanning Calorimetry (DSC). In this work, owing to the high sensitivity of the utilized experimental set-up, a glass transition T_G of 156 K, with a precision equal to 0.3%, and a melting T_M = 220 K have been assessed for 4 µm thick Polydimethylsiloxane (PDMS) films. Performing measurement as a function of time, it was possible to monitor physical aging phenomena, mainly consisting in a change of dielectric properties. As expected, the time evolution of the aging phenomena can be described by a simple logarithmic law.     

Relaxation of Chain Extension of Amorphous PET Fiber Above the Glass Transition Temperature

The average relaxation time for segmental orientation is considerably shorter than that for chain extension1,2, due to the difference of the ability to move between segments and chains. So in certain time intervals, an almost total relaxation of the segmental orientation can take place with little relaxation of global chain extension. Under the controlled conditions1,3, high global chain orientation but nearly random segmental orientation (GOLR) state can be achieved. Through the hot shrinka…     

Glass Transition Temperature and Value of the Relaxation Time at Tg in Vitreous Polymers

Summary: Many works focused on glassy polymers determine values of glass transition temperature (T_g) and an overview of the literature shows that depending on the method used, values of T_g are found different for the same material. In this paper, a review of data collected on different materials are used and interpreted in term of molecular mobility characterized by relaxation time functions. By using three independent experimental procedures (dielectric, thermally depolarized current and calorimetric), we show that the value of the glass transition and the value of the relaxation time at T_g can be correctly determined. It is also shown that the assumption: τ _(Tg) = 100 s is constant, is not correct. The protocol proposed also allows the determination of the value of the fragility index “m” of the glass forming liquid with a great accuracy.     

Glass Transition of Low‐Dimensional Polystyrene

Summary: A unified model is developed for the finite size‐effect on the glass‐transition temperature of polymers, T_g(D), where D denotes the diameter of particles or thickness of films. In terms of this model, T_g depends on both the size and interface conditions. The predicted results are consistent with the experimental evidence for polystyrene (PS) particles and films with different interface situations.

T_g(D) function of free‐standing PS films.     

Relaxation Processes and Glass Transition in Polymer Filled with Nanoparticles

We report the results of the investigations of the influence of filling of polymer with Aerosil nanosize particles on the glass transition and dynamics of the α- and the β-relaxation processes in poly(n-octyl methacrylate) by dielectric spectroscopy and differential scanning calorimetry (DSC). The polymer was filled with hydrophilic and hydrophobic Aerosil particles of 12 nm diameter. In filled polymers the characteristic frequency of the alpha-process was shifted to higher frequencies in comparison with pure bulk polymer at the same temperature. This suggests that the filling of the polymer with nanoparticles has resulted in the shift of its glass transition temperature T_g. This change in T_g was mainly due to the existence of a developed solid particle-polymer interface and the difference in the dynamic behavior of the polymer in the surface layers at this interface compared to the bulk behavior. This result was in agreement with DSC experiments.     

三元乙丙橡胶玻璃化转变温度以上的Sub-Rouse弛豫峰研究

采用动态力学分析法(DMA)研究了三元乙丙橡胶(EPDM)的链段弛豫行为.结果表明随着温度的升高,EPDM样品依次出现了3个内耗弛豫峰,分别是玻璃化转变(α转变)、sub-Rouse模式转变(α'转变)和Rouse模式转变(α″转变).通过分析频率谱中弛豫时间、弛豫强度、耦合参数和形状因子分别随温度的变化,发现在温度为384 K处,sub-Rouse模式转变存在动态转折点TB.而且,转折点的弛豫时间τα'(TB)大约是0.11 s,这与文献报道的PS及PVAc/PEO共混物(10(-0.5±0.5)s)等材料的弛豫时间非常接近,表明该转折点是在特定的弛豫时间产生的.基于链段耦合模型,我们提出此转折点是由低于TB温度时分子间耦合增强引起的.     

Thermal Decomposition and Glass Transition Temperature Study of Poly-p-isopropylstyrene

Thermal decomposition and glass transition temperature studies have been carried out on poly-p-isopropylstyrene (PpiPrS) with a differential scanning calorimeter. The un-decomposed polymer as well as its decomposition products were analyzed by gel permeation chromatography (GPC), infrared spectroscopy (IR) and nuclear magnetic resonance (NMR). During isothermal treatments in the 25–150°C temperature range (α < 3%), the observed increase in M arose primarily from interchain linking between the longer radical-bearing chains. Beyond 315°C (α > 6%), the molecular weight increases result from crosslinking reactions between decomposed polymer and longer undecomposed chains. During interchain linking, the number of isomethyl groups (iso-CH³) increase. In the crosslinking reactions that take place at temperatures beyond 315°C, the number of iso-CH³ and terminal or α-methyl groups (α-CH³) both increase while the number of methylene groups (CH²) decreases. Activation energies of decomposition for various homologs of polystyrene (PS) obey the following order: E_PS > E_PpiPrS > E_PpiPrαMeS ≥ E_PαMeS. A comparison of the Tg_e values of PS with those of PpiPrS, poly-α-methylstyrene (PαMeS) and poly-p-isopropyl-α- methylstyrene (PpiPrαMeS) shows that the presence of the p-isopropyl groups lowers the T_g of PS as well as that of PaMeS by about 30–35° K.     

N-烷基壳聚糖玻璃化转变温度的研究

采用差示扫描量热(DSC)法和热释电流(TSC)法研究了N-烷基壳聚糖的玻璃化转变行为. DSC法中采用二次扫描以消除溶剂和热历史的影响, 并利用物理老化方法来增强N-烷基壳聚糖在DSC曲线上的玻璃化转变区域的热焓吸热峰, 以克服DSC法的不灵敏性. 两种研究方法的结果一致表明, 三种N-烷基壳聚糖的玻璃化转变均发生在110～150 ℃温区内；取代的柔性烷基越大, 玻璃化转变温度(Tg)越低；但N-甲基壳聚糖例外, 其Tg略高于壳聚糖, 空间阻碍在这里起决定的作用.     

两个不同分子量聚苯乙烯混合体系的玻璃化转变

用差示扫描热量法测定了两个分子量不同的聚苯乙烯混合体系的玻璃化转变温度随组成的变化，并与Ｆｏｘ方程，Ｇｏｒｄｏｎ－Ｔａｙｌｏｒ方程和Ｃｏｕｃｈｍａｎ方程的预测结果进行了比较，以验证这些经验方程的适用性。     

聚合物溶液体系玻璃化转变温度与组成之间的关系

大量的文献报道了聚合物溶液体系中聚合物玻璃化转变温度(Tg)与组成之间的关系,这些理论公式大多是基于经典热力学理论,自由体积等理论得到的,这些结果在一些聚合物溶液体系中与实验结果是符合的.在一些聚合物-溶剂体系中,Tg与组成的关系之间出现了反常的"cusp"点,两个玻璃化温度等现象,部分理论值与预测值出现较大偏差,可以用自由体积理论和自聚集理论加以定性的解释.但是依然缺乏普遍的理论来解释和预测上面现象的发生,需要进一步完善和发展聚合物溶液体系中玻璃化转变温度与组成之间的关系.     

玻璃与玻璃化转变

玻璃与玻璃态的应用极其广泛。玻璃化转变是一种典型的非晶液-固转变,当转变发生时体系的结构并没有明显变化,因而我们无法将其归类于已有的任何相变类型。作为凝聚态物理和软物质领域的核心问题,玻璃化转变的研究已有近70年的历史。然而,时至今日,人们还是无法回答玻璃态的本质是什么这一基本问题。本文简述了玻璃态的性质以及伴随玻璃化转变发生的一些基本物理现象,并总结了半个世纪以来一些与玻璃化转变相关的理论,以期加深读者对玻璃及玻璃化转变的认识。     

乳液聚合共聚物玻璃化转变温度及其理论计算

从共聚单体的竞聚率、油相水相分布系数出发,通过动力学模拟计算了丙烯酸丁酯与醋酸乙烯酯乳液共聚物的链结构,并用Johnson公式对其玻璃化转变温度(T_g)进行了理论计算,给出了共聚物T_g及其对应聚合物的重量分布图。发现半连续共聚物有1个T_g,其值随共聚物组成而变化;但一步法共聚物有两个T_g:低温区T_g代表富丙烯酸丁酯共聚物,高温区T_g则代表PVA均聚物。计算结果与实验十分吻合。     

Calorimetric and Dielectric Investigations of Glass Transition in Intermediate Glass-forming Liquid

The calorimetric glass transition and dielectric dynamics of -relaxation in propylene glycol (PG) and its five oligomers (polypropylene glycol, PPG) have been investigated by the modulated differential scanning calorimetry (MDSC) and the broadband dielectric spectroscopy. From the temperature dependence of heat capacity of PPGs, it is clarified that the glass transition temperature (T_g) and the glass transition region are affected by the heating rate. The kinetic changes of PG and PPGs near T_g strongly depend on the underlying heating rate. With increasing the molecular mass of PPGs, the fragility derived from the relaxation time against temperature also increases. The PG monomer is stronger than its oligomers, PPGs, because of the larger number density of the —OH end group which tends to construct the intermolecular network structure. Adam-Gibbs (AG) theory could still hold for MDSC results due to the fact that the dielectric relaxation time can be related to the configurational entropy.This revised version was published online in November 2005 with corrections to the Cover Date.     

Thermoelastic Behaviour of Amorphous Polymers Above and Through the Glass Transition Interval I. Polystyrene

The thermoelastic behaviour of an unfractionated polystyrene was studied in the temperature interval 353–453 K in the regimes of isobaric cooling and of isothermal quasi-adiabatic loading, respectively. The main experimental results can be summarized as follows. 1. In the temperature interval far above the glass transition temperature T_g, both the temperature and volume relaxations of the polystyrene melt after sudden pressure jumps were completely reversible and proved to be simple exponential functions of the time. Therefore, by a straightforward application of Eqs (1) and (2) to the relevant thermoelastic data obtained in a single experimental run one can arrive at the reasonable values of the specific volume, specific heat capacity, thermal diffusivity and heat conductivity of the polymer in the equilibrium melt state. 2. In the temperature interval close to T_g, both the temperature and volume relations of the supercooled polystyrene melt in compression/expansion cycles became markedly asymmetric and non-exponential. The low values of the exponent β in the fractional-exponent Eq. (5) for the volume relaxation suggest a broad spectrum of relaxation times indicating the high degree of coupling between different mechanisms of the molecular motions involved. This revised version was published online in July 2006 with corrections to the Cover Date.