Thermal conductivity behaviour of polymers around glass transition and crystalline melting temperatures

The thermal conductivity of five semi-crystalline and four amorphous polymers was determined within a wide range of temperature, starting at room temperature and going up to temperatures above the polymer melting point (Tm) for semi-crystalline polymers or above the glass transition temperature (Tg) for amorphous polymers. Two transient techniques were employed in the experimental investigation: the hot wire technique for the group of amorphous polymers, and the laser flash technique for the semicrystalline polymers. As expected, the experimental results show that Tg exerts a measureable influence on the thermal conductivity of amorphous polymers. In the case of semi-crystalline polymers, a singular behaviour of the thermal conductivity is observed within the Tm range. In order to explain the anomalous behaviour, the influence of these transition temperatures on the thermal conductivity behaviour with temperature has been analysed in terms of a phonon conduction process and temperature variations of specific heat and modulus of elasticity of the analyzed polymers.     

Low glass transition temperature fluorocarbon ether bibenzoxazole polymers

Fluorocarbon ether bis(o-aminophenol) monomers were prepared by a multistep synthetic route based on the copper-promoted coupling of 4-iodophenyl acetate with 1,8-diiodoperfluoro-3,6-dioxaoctane, 1,11-diiodoperfluoro-3,9-dioxaundecane, 1,14-diiodoperfluoro-5,10-dimethyl-3,-6,9,12-tetraoxatetradecane, and 1,17-diiodoperfluoro-3,6,9,15-tetraoxaheptadecane. Acetic acidpromoted polycyclocondensations of the monomers with long-chain fluorocarbon ether-diimidate esters and -dithioimidate esters led to linear fluorocarbon ether-bibenzoxazole polymers soluble in 1,1,1,3,3,3-hexafluoroisopropanol and 1,1,2-trichloro-1,2,2-trifluoroethane. Polymer structures were verified by elemental and infrared spectral analysis. The polymers were rubbery gums and could be obtained in the inherent viscosity range of 0.20–0.79 dlg^?1. Selection of monomers governed the glass transition temperatures of the resultant polymers. As expected, the polymers exhibited lower glass transition temperatures with increased fluorocarbon ether content, a minimum value of ?58°C being achieved. None of the polymers exhibited crystalline melt temperatures. Based on thermogravimetric analysis data, the thermooxidative stability of the polymers tended to decrease with increased fluorocarbon ether content. Onset of breakdown during thermogravimetric analysis in air occurred in the 350–400°C range. Isothermal aging of the polymers in air indicated good thermooxidative stability at 260°C; only 5% weight loss was recorded after 200 hr.     

Increased glass transition temperature in motionally constrained semicrystalline polymers

A large number of experimental results in the literature support and illuminate a model of behavior of chains and chain segments in the amorphous phase of semicrystalline polymers connecting the elevation of the glass transition temperature (T_g) above its normal value to several kinds of motional restrictions imposed on the chains and parts thereof. Accordingly, polymer chain, chain-segment and chain-fragment motions of all kinds comprise one or more torsions around main-chain bonds from one stable conformation to another, known as rotational isomerizations. When impediments are placed in front of thermal fluctuations and larger transversal and longitudinal motions of polymer chains, segments and shorter fragments in the amorphous phase, and the motions are thus restricted, the glass transition temperature is elevated relative to that of the same amorphous phase in the bulk under normal conditions. The obstructions may prevent either the onset of rotational isomerizations or of their completion once started. The completion of the torsional isomerizations and larger motions may be prevented by eliminating the free spaces necessary to accommodate the volumes of the interconverting chain fragments and segments even when they move in concert, or by preventing the creation of such free spaces. Another way to hinder the completion of such motions is by the introduction into the system of many rigid walls and other interfaces with strong attractive interactions with the polymer, that by geometrical constraints and attractive interactions suppress the rotational and larger motions and prevent their completion. Elimination of the necessary free volume is achievable by the application of compressive pressure, while the introduction of rigid attractive walls may be accomplished by the incorporation of crystallites, as in semicrystalline polymers, or by the addition of rigid finely comminuted foreign additives with very large surface areas or confining voids with high tortuosity. It is believed that motional restrictions imposed on the amorphous phase by the growth faces of polymer crystallites, especially in oriented semicrystalline polymers, are more effective than the restrictions imposed by the fold surfaces of these crystallites. The prevention of the onset of rotational isomerizations and larger motions may be achieved by stretching the polymer chains and chain segments in the amorphous phase and, by one means or another, pinning down the taut chains such that essentially all their rotational isomers are in the trans conformation: they cannot interconvert to the gauche conformation since it requires the chain’s end-to-end distance to decrease. Parallel alignment of relatively taut chain-segments may impose additional geometrical restrictions on both the onset and completion of rotational isomeric torsions and, of course, on longer-range motions. In all cases, the T_g of the motionally constrained parts of the amorphous phase, especially in semicrystalline polymers, is expected to rise. It is likely that the characteristic length associated with transversal motions and their suppression is R_c, the spatial distance between entanglements, which is of the same size scale, and may be the same as the tube diameter of the reptation model. Special emphasis was placed in this work on the semicrystalline polymers poly (ϵ-caprolactam) (nylon-6) and poly (ethylene terephthalate) (PET). © 1998 John Wiley & Sons, Ltd.     

Tensile strength of oriented polymers below the glass transition temperature

A theory of tensile strength, based on the observation of cracks in specimens strained to breaking, is formulated. The treatment involves the assumption that a crack grows to a critical size by a nucleation process. When this critical size is exceeded the crack becomes unstable and propagates spontaneously to produce rupture. By comparing the predicted and measured strength, one can estimate the magnitude of the stress concentration factor in fibers. An interpretative analysis of experimental data obtained at various strain rates indicates that the resulting changes in tensile strength are due primarily to the changes in modulus.     

Thermally stimulated current and DSC studies of the broadened glass transition in liquid crystalline polymers

The high sensitivity of the thermally stimulated current, thermal sampling (TS) method is emphasized in a study of the breadth of the glass transition in several liquid-crystalline polymers (LCPs). Differential scanning calorimetry (DSC) was performed on all samples to further quantify the glass transition regions. For “random” copolyester LCPs with widely varying degrees of crystallinity, including highly amorphous samples, very broad glass tran-sition regions were observed. One semicrystalline alternating copolyester and a series of semicrystalline azomethine LCPs were studied as examples of structurally regular polymers. These exhibited relatively sharp glass transitions more comparable to ordinary isotropic amorphous or semicrystalline polymers. The broad glass transitions in the random copolyesters are attributed to structural heterogeneity of the chains. In one example of a moderate-crystallinity random copolyester LCP (Vectra), glass transitions ranging up to ca. 150°C in breadth were determined by the thermal sampling (TS) method and DSC. In other lower crystallinity copolyester LCPs, the main glass transition temperature as determined by DSC was comparable to that determined by TSC although cooperative relaxations of a minor fraction of the overall relaxing species were detected well below the main T_g, by the TS method and not by DSC. Rapid quenches from the isotropic melt to an isotropic glass were possible with one LCP. The anisotropic and isotropic glassy states for this LCP were found to have the same breadth of the glass transition as was determined by the TS method, although TSC and DSC show that T_g is shifted downward by ca. 15°C in the anisotropic glass as compared to the isotropic glass. © 1993 John Wiley & Sons, Inc.     

New simple method of measuring the surface glass transition temperature of polymers

A lap‐shear joint mechanical testing method has been probed to measure the surface glass transition temperature (T) of the thick bulk films of high‐molecular‐weight polymers. As T, the temperature transition “occurrence of autoadhesion–nonoccurrence of autoadhesion” has been proposed. The influence of chain flexibility, of molecular architecture, of polymer morphology, and of chain ends concentration on the T has been investigated. The correlation between the reduction in T with respect to the glass transition temperature of the bulk (T) and the intensity of the intermolecular interaction in the polymer bulk in amorphous polymers has been found. The effect of surface roughness on T has been discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 2012–2021, 2010     

Elevation of the glass transition temperature in flexible‐chain semicrystalline polymers

In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T_g) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine T_g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T_g is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007     

Interactions in a blend of two polymers greatly differing in glass transition temperature

Blends of poly(N‐methyldodecano‐12‐lactam) PMDL with poly(4‐vinyphenol) PVPh have been studied by the DSC and ATR FTIR methods. The difference in glass transition temperature T_g between the components is 206 °C. A single composition‐dependent T_g suggests miscibility of the system, that is, homogeneity on the scale of about 10 nm. Fitting of the equation of Brostow et al. to the T_g data indicates relatively strong specific interactions and high complexity of the system. The Schneider's equation applied separately to low‐ and high‐PVPh regions provides good agreement with experiment; the calculated curves cross at the point of PVPh weight fraction 0.27. In the low‐PVPh region, the analysis indicates weak interactions with predominance of segment homocontacts and strong involvement of conformational entropy. In the high‐PVPh region, strong specific interactions predominate and entropic effects are suppressed. Composition dependences of the heat capacity difference at T_g and the width of glass transition indicate strong interactions in the system and existence of certain heterogeneities on segmental level, respectively. According to ATR FTIR, hydrogen bonds between PVPh as proton donor and PMDL as proton acceptor induce miscibility in blends of higher PVPh content (above about 0.28 weight fraction). In low‐PVPh blends, it is conformational entropy that enables intimate intermolecular mixing. Hydrogen bonds adopt several (distorted) geometries and are on average stronger than average hydrogen bonds formed in self‐associating PVPh. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

On the relationship between microhardness and glass transition temperature of some amorphous polymers

On the basis of microhardness (H) data measured at room temperature only for a number of polymers in the glassy state, a linear correlation between H and the glass transition temperature T_g has been found (H = 1.97T_g − 571). By means of this relationship, the deviation of the H values from the additivity law for some multicomponent and/or multiphase polymeric systems can be accounted for. The latter usually contains a liquidlike soft component and/or phase with T_g below room temperature. A completely different deformation mechanism in comparison to systems with T_g above room temperature is invoked. A novel expression for the hardness of polymers in terms of crystallinity of the single components and/or phases, the T_g values, and the mass fraction of each component is proposed. This expression permits the calculation of (i) the room‐temperature H value of amorphous polymers, mainly containing single bonds in the main chain, provided T_g is known, and of (ii) the contribution of the soft liquidlike components (phases) to the hardness of the entire multiphase system. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1413–1419, 1999     

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers

A generalized theory for the glass transition temperature of crosslinked and uncrosslinked polymers has been developed, which takes into account the influences of end groups, branching, and crosslinking, and their functionality distribution. DiBenedetto's theory was found to correctly characterize the influence of crosslinks on the glass temperature. Normalized to constant crosslink functionality, the crosslink constant is a universal parameter suggesting that the entropic theory of glasses is applicable to crosslinked systems. Data on linear polymers and networks from the crosslinking of polymer chains, vinyl/divinyl-copolymers and step-growth polymers, such as polyurethanes, amine-cured epoxies, or inorganic glasses, are presented.     

The proper glass transition temperature of amorphous polymers on dynamic mechanical spectra

Glass transition is crucial to the thermal and dynamical properties of polymers. Thus, it is important to detect glass transition temperature (T _g) with a sensitive and proper method. Dynamic mechanical analysis (DMA) is one of the most frequently used methods to determine T _g due to its advantage of high sensibility. However, there is controversy in the past literatures to determine the proper glass transition temperature among three transition temperatures, i.e., T _g1, T _g2 and T _g3 in the dynamic mechanical spectra, which correspond to the temperature abscissa of intersect value of two tangent lines on storage modulus (E′), the peak of the loss modulus (E″) and the peak of the loss tangent (tan δ). In this work, these three transition temperatures were compared with the glass transition temperature determined by DSC (T _gDSC). Based on the discussion of different modes of molecular motion around the glass transition region, it is demonstrated that T _g1 and T _g2 have the same molecular mechanism as T _gDSC, i.e., local segmental motion which is enthalpic in nature and determines the proper glass transition temperature, while T _g3 is assigned to the transition temperature of entropic Rouse modes, thus cannot be used as the proper glass transition temperature.     

The abnormal behavior of polymers glass transition temperature increase and its mechanism

In terms of the classical theory in textbooks, the two components with phase separation in a binary polymer blend will, depending on their compatibility, have their respective Tg get closer or remain in their original values. According to the classical theory, the Tg of plastic component shall remain unchanged or move toward the lower Tg of rubber component in a rubber/plastic blend. However, ultra-fine full-vulcanized powdered rubber (UFPR) with a diameter of ca. 100 nm can simultaneously increase the toughness and the Tg of plastics, which is abnormal and is difficult to explain by classical theory. In this feature article, the abnormal behavior and its mechanism are discussed in detail.     

In situ measurement of the glass transition temperature of polymers with compressed fluid diluents

In situ measurement of the creep compliance of poly(methyl methacrylate) (PMMA) and poly(ethyl methacrylate) (PEMA), equilibrated with a pressurized CO₂ phase, is used to determine the glass transition temperature. Corrections due to dilation of the polymer by CO₂ as well as the buoyancy are assessed. Both polymer systems exhibit a recently discovered phenomenon, retrograde vitrification, in which a liquid polymer becomes a glass with an increase in temperature. The experimental results are predicted semi-quantitatively in terms of the temperature and pressure effects on the solubility of the compressed fluid in the polymer. © 1994 John Wiley & Sons, Inc.     

Determination of the glass transition temperature

The definition of the glass transition temperature, T _g, is recalled and its experimental determination by various techniques is reviewed. The diversity of values of T _g obtained by the different methods is discussed, with particular attention being paid to Differential Scanning Calorimetry (DSC) and to dynamic techniques such as Dynamic Mechanical Thermal Analysis (DMTA) and Temperature Modulated DSC (TMDSC). This last technique, TMDSC, in particular, is considered in respect of ways in which the heterogeneity of the glass transformation process can be quantified.     

Light-scattering photography of polymers near the glass transition

Laser light scattered from a block of polystyrene is observed to have a pattern of bright and dark regions called speckle. We observe that the speckle pattern has a lifetime consistent with that of the average relaxation lifetime 〈τ〉 of the intensity fluctuations of the medium as determined by photon correlation spectroscopy. In order to study the pattern of the scattered light, a series of photographs was taken at a variety of exposure times. For each series, the value of the average relaxation lifetime is defined by the temperature and pressure of the polystyrene sample. When the value of 〈τ〉 is short relative to the exposure time, the photograph displays only a random pattern of exposed grains. This is due to the large number of fluctuations that have occurred during the exposure time with random phases relative to one another. As the average relaxation time is increased at a constant exposure time, the speckle pattern appears when the value of 〈τ〉 becomes comparable to the exposure time. The phenomenon of laser speckle allows the time scale of the slowly relaxing fluctuations near the glass transition to be visualized. A digitized series of such pictures could be analyzed to obtain the average relaxation time for the fluctuations, just as in normal photon correlation spectroscopy where the intensity of one coherence area is measured as a function of time.