Physical Aging of Maltose Glasses as Measured by Standard and Modulated Differential Scanning Calorimetry

The physical aging characteristics of maltose glasses aged at two temperatures below the glass transition temperature, T_g, (T_g-10°C and T_g-20°C) from 5 to 10 000 min were measured by standard differential scanning calorimetry (SDSC) and modulated differential scanning calorimetry (MDSC). The experimentally measured instrumental T_g, the calculated T_g, and the excess enthalpy values were obtained for aged glasses using both DSC methods. The development of excess enthalpy as a function of aging time, as measured by both SDSC and MDSC, was fit using the Cowie and Ferguson and Tool-Narayanswamy-Moynihan models. The change in the T_g values and the development of the excess enthalpy resulting from physical aging measured by the two DSC methods are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Influence of Drawing in Hot Water on The Morphological Properties of Pet Films as Measured by DSC and Modulated DSC

PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition is studied by MDSC which allows to access the glass transition temperature T _g and the variations of ΔC _p=C _p1−C _pg (difference between thermal capacity in the liquid-like and glassy states at T=T _g). Variations of T _g with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed with regard to the structure engaged in these materials. The increments of ΔC _p at T _g are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’ of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Thermal characterization and electrical properties of Fe-modified cellulose long fibers and micro crystalline cellulose

Thermal properties of polylactic acid (PLA) filled with Fe-modified cellulose long fibers (CLF) and microcrystalline cellulose (MCC) were studied using thermo gravimetric analysis (TG), differential scanning calorimetry, and dynamic mechanical analysis (DMA). The Fe-modified CLFs and MCCs were compared with unmodified samples to study the effect of modification with Fe on electrical conductivity. Results from TG showed that the degradation temperature was higher for all composites when compared to the pure PLA and that the PLA composites filled with unmodified celluloses resulted in the best thermal stability. No comparable difference was found in glass transition temperature (T _g) and melting temperature (T _m) between pure PLA and Fe-modified and unmodified CLF- and MCC-based PLA biocomposites. DMA results showed that the storage modulus in glassy state was increased for the biocomposites when compared to pure PLA. The results obtained from a femtostat showed that electrical conductivity of Fe-modified CLF and MCC samples were higher than that of unmodified samples, thus indicating that the prepared biocomposites have potential uses where conductive biopolymers are needed. These modified fibers can also be tailored for fiber orientation in a matrix when subjected to a magnetic field.     

Creep behavior of amorphous ethylene–styrene interpolymers in the glass transition region

The viscoelastic behavior of amorphous ethylene–styrene interpolymers (ESIs) was studied in the glass transition region. The creep behavior at temperatures from 15°C below the glass transition temperature (T_g) to T_g was determined for three amorphous ESIs. These three copolymers with 62, 69, and 72 wt % styrene had glass transition temperatures of 11, 23, and 33°C, respectively, as determined by DMTA at 1 Hz. Time–temperature superposition master curves were constructed from creep curves for each polymer. The temperature dependence of the shift factors was well described by the WLF equation. Using the T_g determined by DMTA at 1 Hz as a reference temperature, C₁ and C₂ constants for the Williams, Landel, and Ferry (WLF) equation were calculated as approximately 7 and 40 K, respectively. The master curves were used to obtain the retardation time spectrum and the plateau compliance. The entanglement molecular weight obtained from the plateau compliance increased with increasing styrene content as 1,600, 1,870, and 2,040, respectively. The entanglement molecular weight of the ESIs was much closer to that of polyethylene (1,390) than to that of polystyrene (18,700); this was attributed to the unique chain microstructure of these ESIs with no styrene–styrene dyads. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2373–2382, 1999     

Isothermal crystallization of concentrated amorphous starch systems measured by modulated differential scanning calorimetry

The slow isothermal crystallization of concentrated amorphous starch systems is measured by Modulated Differential Scanning Calorimetry (MDSC). It can be followed continuously by the evolution (stepwise decrease) of the MDSC heat capacity signal (Cp), as confirmed with data from X-ray diffractometry, Dynamic Mechanical Analysis, Raman spectroscopy, and conventional Differential Scanning Calorimetry. Isothermal MDSC measurements enable a systematic study of the slow crystallization process of a concentrated starch system, such as a pregelatinized waxy corn starch with 24 wt % water and 76 wt % starch. After isothermal crystallization, a broad melting endotherm with a bimodal distribution is observed, starting about 10°C beyond the crystallization temperature. The bulk glass transition temperature (T_g) decreases about 15°C during crystallization. The isothermal crystallization rate goes through a maximum as a function of crystallization time. The maximum rate is characterized by the time at the local extreme in the derivative of Cp (t_max), or by the time to reach half the decrease in Cp (t_1/2). Both t_max and t_1/2 show a bell-shaped curve as a function of crystallization temperature. The temperature of maximum crystallization rate, for the system studied, lies as high as 75°C. This is approximately 65°C above the initial value of T_g. Normalized Cp curves indicate the temperature dependence of the starch crystallization mechanism. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2881–2892, 1999     

On the theory of thermal density fluctuations in the glassy state: Application to poly(vinyl acetate)

A theory of the liquid state, suitably modified for the glass, contains a characteristic structure functionh, which represents a free volume fraction. As shown previously by means of experimental pressure-volume-temperature studies,h retains finite, nonvanishing temperature and pressure coefficients upon passing through the glass transition. These results are now employed to compute the mean-square thermal density fluctuations in poly(vinyl acetate). AboveT _g , the result attests again to the satisfactory quantitative performance of the equilibrium theory. BelowT _g , two glasses formed at low and elevated pressures, respectively, are considered under quasi-equilibrium conditions. The results show the anticipated initial accord with the approximation proposed by Fischer and Wendorff, involving the isothermal compressibility of the liquid atT _g . The theory delineates the increasing departures with decreasing temperature observed in the literature. We comment finally on the trend of the fluctuations on approaching absolute zero. Explicit low temperature calculations remain to be undertaken.Dedicated to Professor Dr. F. H. Müller.     

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

The nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (T_g) on film thickness. Sizeable reduction of T_g was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of T_g on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the T_g or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

高聚物玻璃化转变温度和分子参数的关系 Ⅰ.高聚物链柔性与Tg的关系

 本文采用晶格模型,以动力学链段长度作为统计单元大小,推导了高聚物玻璃化温度T_8和链静态刚性因子σ²(T₈),链动态刚性因子β(T₈)以及聚合度DP等分子参数之间的关系。具体讨论了链柔性对T₈的影响。理论预测和几十种聚合物的实验数据能较好吻合,分析结果表明T₈值基本上取决于高聚物链σ(T₈)大小。     

3D diamond‐containing nanocomposites based on hybrid polyurethane–poly(2‐hydroxyethyl methacrylate) semi‐IPNs: Composition‐nanostructure‐segmental dynamics‐elastic properties relationships

Nanostructure, glass transition dynamics and elastic properties were studied in the 3D nanodiamond‐containing composites based on polyurethane‐poly(2‐hydroxyethyl methacrylate) semi‐interpenetrating polymer networks (PU‐PHEMA semi‐IPNs), neat PU or PHEMA matrices. Nanodiamond (ND) content in the nanocomposites varied from 0.25 to 3 wt %. Combined differential scanning calorimetry/ laser‐interferometric creep rate spectroscopy/atomic force microscopy approach was utilized. A large impact of small 3D ND additives on PU‐PHEMA networks' dynamics and properties was revealed under conditions when an average inter‐particle distance L exceeds by far gyration radius R_g. The pronounced heterogeneity of glass transitions' dynamics and two opposite effects were observed. The main effect was a strong suppression of PHEMA glass transition dynamics at 90–180 °C, with the enhancement of creep resistance and threefold to sixfold increasing modulus of elasticity. The peculiarly crosslinked structure of nanocomposites, due to double covalent hybridization, resulted in low rheological percolation threshold, and a synergistic effect in dynamics was observed. Less pronounced effect of accelerating dynamics in the temperature region between β‐ and α‐transitions in PHEMA was associated with dynamics in domains with loosened molecular packing. The distinct physical limit for “anomalous” decreasing T_g is predicted in terms of the notion of the common segmental nature of α‐ and β‐relaxations. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1696–1712, 2008     

Effects of thermal treatment on biaxially oriented poly (ethylene terephthalate). III. Creep behavior following various thermal histories

Elongational creep measurements were carried out on a biaxially oriented poly (ethylene terephthalate) (PET) film parallel to, orthogonal to, and at 45° to the principal optic axis. Measurements made after various thermal treatments which were intended to stabilize the physical state of the PET were shown to be ineffective. Samples were annealed at 140°C for 12 days and aged at 95°C for over 24 days before measurement without success. Thermal cycling between 41 and 91°C which was also employed to stabilize the mechanical response also failed. Significant deceleration of the creep rate caused by densification of amorphous regions of the samples during storage below the glass temperature T_g is illustrated. Because of physical aging below T_g and morphological changes occurring above T_g during the various thermal treatments and histories, time-scale shift factors were found to be not unique.     

The linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906     

The evolution of the viscoelastic retardation spectrum during the development of an epoxy resin network

The evolution of the viscoelastic behavior of an epoxy resin at various stages of curing has been followed with the changes in the retardation spectrum. The creep J(t) and recoverable creep compliance J_r(t) curves of the neat epoxy resin Epon l00lF (Shell) were determined at temperatures between 30 and 77°C. The viscosity decreased over 8 orders of magnitude as the temperature was increased. Specimens with eight stages of network development were prepared by reacting all of the epoxy resin's oxirane rings with amine hydrogens from varying ratios of a monofunctional amine (methyl aniline) and a tetrafunctional amine 4,4′-diamino diphenyl sulfone (DDS). Preparations in which 25, 35, and 40% DDS were used did not result in a molecular network, so they were viscoelastic liquids. With 45% DDS, the product had a nascent network and was judged to be just beyond the point of incipient gelation. The remaining preparations from 0.50, 0.60, 0.70, and 1.0 DDS yielded tighter less compliant molecular networks. The creep and recoverable compliance curves were measured over a range of temperatures above the glass transition temperature, T_g. They were reduced to T_g, and retardation spectra L(ln τ) were calculated.     

Small Strain Mechanical Properties of Latex-Based Nanocomposite Films

A waterborne latex-based technique, in which functionalized laponite is attached to PS and acrylic latex particles, is used to prepare films containing up to 50 wt% laponite. At high laponite contents this leads to a cellular arrangement of the laponite-rich layers, concentrated at the latex particle interfaces. MDSC shows that a significant proportion of the organic matrix does not contribute to the glass transition. However, this “rigid” matrix fraction arises from intercalation of the laponite stacks, and cannot account for the large increases in global stiffness in the rubbery state (T > T_g) on laponite addition. The mechanical response for T > T_g is therefore discussed in terms of a four-phase structure, in which the intercalated laponite stacks embedded in a matrix with a relatively high rubbery modulus form a cellular structure, which is in turn embedded in a matrix whose modulus is closer to that of the bulk polymer. The importance of the cellular arrangement is underlined by the much lower rubbery modulus observed by DMA in specimens produced by deforming the original films in plane strain compression to produce oriented textures with relatively little connectivity between the laponite-rich layers.     

Effect of water content on the glass transition and textural properties of hazelnut

This study is based on the assumption that the change in the texture of hazelnut, induced by water sorption or desorption, can be derived from the glass transition. No previous study has investigated the glass transition properties of hazelnuts. This study aimed to investigate the effect of water content on the glass transition and textural properties of a roasted hazelnut product. Water content of the sample was adjusted in various relative humidity conditions, and the mechanical glass transition temperature T_g was investigated using thermal rheological analysis (TRA), a type of thermomechanical analysis. The TRA curve exhibited a clear force drop induced by the glass transition, and mechanical T_g of the samples was determined. Water plasticizing effect caused mechanical T_g to decrease as the water content increased. The reduction in T_g was analyzed using the Gordon–Taylor equation and linear equation, and the critical water contents (water content at mechanical T_g?=?25 °C) were obtained. The fracturing properties of the hazelnut changed from brittle to ductile at the critical water contents. This suggested that the change in the texture of hazelnut can be characterized by the glass transition.

     

Investigations of the Sub-Ambient Transitions in Frozen Sucrose by Modulated Differential Scanning Calorimetry (MDSC®)

40% w/w sucrose/water solutions were analyzed by Modulated Differential Scanning Calorimetry [1] in the sub-ambient temperature region. At these temperatures, the solutions exhibit a complex, two-step thermal event. The lower-temperature event is believed to be the glass transition of the amorphous sucrose phase. The nature of the higher-temperature event is the subject of controversy. This event has been shown to have distinct second-order characteristics, and as such is believed to be a second T_g. Others feel that this event is the onset of melting. The temperature region between these events contains a devitrification exotherm. Through the use of MDSC, both in scanning and stepwise quasi-isothermal modes, improved sensitivity and resolution of MDSC provides new insight into the nature of these transitions.This revised version was published online in November 2005 with corrections to the Cover Date.     

Effects of thermal treatment of biaxially oriented poly(ethylene terephthalate)

Two distinguishable effects of thermal exposure of biaxially oriented poly(ethylene terephthalate) (PET) have been observed in the temperature range from room temperature to 140°C. Upon heating above the glass transition temperature T_g of the film an irreversible shrinkage of a few percent occurred with a concomitant decrease in the rate of creep. Some loss of orientation in the noncrystalline phase with an attendant slight increase in density is believed to be responsible. Since the film was anisotropic in its plane, different amounts and rates of shrinkage were observed along with differing thermal expansion coefficients in various directions relative to the primary optic axis. Upon cooling the 50% crystalline PET from above T_g to lower temperatures, reversible “physical aging” was observed. Creep rates were found to decrease with the residence time below T_g. As with purely amorphous polymers, the effects of the aging are removed by heating the specimen above T_g where the density of the amorphous phase achieves equilibrium values.     

Bulk and shear rheology of a symmetric three‐arm star polystyrene

The bulk and shear rheological properties of a symmetric three‐arm star polystyrene were measured using a self‐built pressurizable dilatometer and a commercial rheometer, respectively. The bulk properties investigated include the pressure–volume–temperature behavior, the pressure‐dependent glass transition temperature (T_g), and the viscoelastic bulk modulus and Poisson's ratio. Comparison with data for a linear polystyrene indicates that the star behaves similarly but with slightly higher T_gs at elevated pressures and slightly higher limiting bulk moduli in glass and rubbery states. The Poisson's ratio shows a minimum at short times similar to what is observed for the linear chain. The horizontal shift factors above T_g obtained from reducing the bulk and shear viscoelastic responses are found to have similar temperature dependence when plotted using T ? T_g scaling; in addition, the shift factors also exhibit a similar temperature dependence to linear polystyrene. The retardation spectra for the bulk and shear responses are compared and show that the long time molecular mechanisms available to the shear response are unavailable to the bulk. At short times, the two spectra have similar slopes, but the short‐time retardation spectrum for the shear response is significantly higher than that for the bulk, a finding that is, as yet, unexplained. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. I. Self-organization of cationic polymer particles on alkylated glass plates

Monodisperse, cationic polymer particles bearing quaternary ammonium groups effectively self-organized on hydrophobic solid substrates such as alkylated glass plates and polymer films to form particle monolayers. With an increase of the particle surface charge density, the surface coverage decreased and the morphology of particle monolayers changed from aggregated type to dispersed type. The dispersed type of particle monolayers having a relatively regular particle distance was formed at higher temperature. The self-organization behaviors on alkylated glass plates were different from those on unmodified glass plates through electrostatic interaction. The formation of particle monolayers on alkylated glass plates occurred only over a certain latex concentration range in contrast with that on unmodified glass plate. The adhesive strength of particle monolayers was enhanced by annealing at temperatures above the glass transition temperature (T _g) of the particles. Lens-shaped particle monolayers were fabricated by annealing the dispersed type of particle monolayers.     

Viscoelastic properties of poly(methyl methacrylate) with high glass transition temperature by lithium salt addition

The effect of ion‐dipole interaction between lithium cations and oxygen atoms in poly(methyl methacrylate) (PMMA), which leads to the great enhancement of glass transition temperature (T_g), on the linear viscoelastic properties is studied using binary blends of PMMA and lithium trifluoromethanesulfonate (LiCF₃SO₃). The strong interaction at low temperature leads to the high modulus in the glassy region even near T_g. The interaction becomes weak as increasing the temperature. Consequently, the rheological terminal region is clearly detected without a marked enhancement of steady‐state compliance, although the zero‐shear viscosity increases by the LiCF₃SO₃ addition. The result indicates that the crosslinking due to the ion‐dipole interaction has a lifetime that decides the longest relaxation time. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2388–2394