DSC monitoring of supermolecular structure damage of polyethylene products

Investigated materials were produced from polyethylene of low density (PE-LD) and a filler: carbon black or chalk. A part of the samples was kept in ambient temperature over several months in order to study the effect of material ageing. The qualitative conclusions were drawn based on a precise analysis of shape of DSC curves and the basic investigations of PE morphologies known from literature. The influence of the thermal history and the filler contents on the amount and kind of crystal phase was established. Additionally, the effect of the measurement technique, in our case it was positron annihilation lifetime spectroscopy (PALS), on the morphological feature changes was found. The paper serves selected examples of problems solved by DSC. However, from the other hand, the authors discuss critically the opportunity given by this method.     

Thermal Analysis of the Conformational Disorder in SCLC Polymers with Rigid Backbone Glass transition studies

Bis [(ω-(4′-cyanobiphenyl)-4-yl)oxy-n-alkyl]norborn-5-ene-2,3-dicarboxylate was polymerised via ring opening metathesis polymerisation (ROMP). Two disubstituted polynorbornene derivatives both of cis configuration with different length of the side-chain were studied. Differential scanning calorimetry (DSC) was used to study the effect of thermal history on the assignment of the glass transition event associated with the biaxial orientation of a smectic phase. Glass transition temperatures, the change of isobaric specific heats at T_g and the enthalpies of isotropisation were calculated. The DSC traces only show the classic step-wise change in T_g in some cases, giving the evidence that the amorphous domains are constrained and highly restricted in movement due to the morphology developed as a result of the biaxial stretching. Based on the literature data of mono- and disubstituted polynorbornene derivatives and our calorimetric experiments, the shape of T_g dependence on number of (CH₂) units is interpreted. The origin of this shape is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic parameters from thermogravimetric study of used rubber granulates–polyurethane composites

From the TG data of rubber granulates, different polyurethane and composites it can be seen that the thermal decomposition for the rubber granulate and all of the composites start above 520 K. Two major mass losses for the rubber granulates and majority of the composites were observed and thermal decomposition is essentially complete by ~820 K. The changes of activation energies of lower and higher temperature decomposition, calculated according to the different equations were observed for a priori assumed first-order reaction for devolatilisation. Differences between determined and calculated results could suggest a possible reaction between polyurethane agents and rubber granulate during the composites formations.     

Effect of Supermolecular Structure Changes on the Glass Transition of Polymer

The idea of two miscellaneous amorphous fractions in one polymeric system is under consideration. A new factor influencing the glass transition and a relaxation observed in polymers exhibiting different supermolecular structures is discussed. In order to clarify the specific arrangement of backbone chains two types of polymers were taken under investigation: semicrystalline poly(4-methyl-1-pentene) and a liquid-crystalline poly(norbornene) derivative. In both cases,it was possible to find two well separated glass transitions. Differential scanning calorimetry and dynamic mechanical thermal analysis were used to study the effect of thermal history on the assignment of the glass transition event associated with two miscellaneous amorphous phases. The problem of molecular arrangement in the amorphous phases is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamic Mechanical Relaxation in the Opaque and Transparent Poly(4-Methyl-1-Pentene) Films The glass transition zone

The thermomechanical properties of opaque and transparent polymer films of a solution of poly(4-methyl-1-pentene) (PMP) in cyclohexane and carbon tetrachloride obtained by casting on teflon and glass plates were investigated. The dynamic mechanical thermal analysis was applied in a frequency range from 0.01 to 100 Hz. The curves of loss tangent vs. temperature varied depending on the sample thermal history. The first part of these curves could relate to the backbone α relaxation into the unperturbed amorphous phase while the next relaxation could result from the backbone α relaxation into amorphous phase perturbed by the presence of the crystal domains. The Arrhenius plots of the first relaxation show a stronger curvature found in each of the transparent samples indicating strong dependency on specific volume. The second one in the case of transparent films and the first one for opaque samples might be approximate to straight lines. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermodynamics and structure of the ordered amorphous phase in polymers

Experimental data are analysed to show that the activation enthalpy and the structural entropy appear to follow empirical relations for structural relaxation of polymeric systems. The relations indicate that melting temperature, glass transition temperature, relaxation time, coefficient of thermal expansion of free volume are interrelated in polymers. The parameters of structural relaxations, measured by mechanical and dielectric spectroscopies, are reviewed for polyethylene, poly(4-methyl-1-pentene) and a liquid-crystalline polynorbornene derivative. The thermodynamic parameters obtained from calorimetric measurements, are reported for zero heating rate extrapolation and they are used in the empirical relation, which combines the Arrhenius and the Vogel-Fulcher formulae.     

On the α relaxation of the constrained amorphous phase in poly(ethylene)

The idea of coexistence of two different amorphous fractions in semi-crystalline poly(ethylene) (PE) is taken into consideration. Differential scanning calorimetry, dynamic mechanical thermal analysis and positron annihilation were used to study the effect of supermolecular structure on the glass transition associated with ‘semi-ordered’ amorphous phase. The arrangement of chains, located in this phase, is forced internally by the presence of crystalline domains. Influence of drawing, irradiation and annealing on thermodynamic and structural parameters was investigated. In a series of experiments the correlation between the glass transition, the α relaxation and the free volume of a semi-crystalline system was found. In order to better describe the relaxation, existence of two types of amorphous fractions was assumed. The relaxations are treated as a long-scale motion of macromolecule segments located into the ‘semi-ordered’ amorphous regions, or into the ‘real’ amorphous regions, and are labelled as α_c and α_g, respectively. The magnitudes of the loss peaks were quite sensitive to the free volume, which was found to increase by drawing and decrease by irradiation. Two different DSC traces were recorded for both glass transitions.