N.F. Bakeyev

Melting temperature, heat of melting and crystallinity determinations on polyethylene by differential scanning calorimetry

The differential scanning calorimeter DSM-2 has been used to study the melting of PE samples having different molecular weight, branching intensities, thermal histories and physical states (powders, granules, films with and without orientation), as well as a range of densities (0·909-0·998 g/cm3). The melting temperature Tm and the

X-ray structural analysis of amorphous regions in oriented films of linear polyethylene☆

Abstract Regularities of structural transitions and density changes occurring in amorphous regions in linear PE films during orientational drawing and reversible deformation of oriented specimens have been investigated by recording the low angle X-ray scattering in absolute units and in relative units. It is shown that depending on the drawing ratio the density of amorphous regions in the linear PE films is 0·86–0·92 g/cm3. A quantitative approach to structural description of the amorphous regions is proposed.

Effect of annealing under high pressure on the structure of oriented polyethylene

0 ~ of t he ma in me thods of changing the s t ruc tu re of crystal l ine po lymers is b y hea t t r e a t m e n t a t h igh pressure. M a n y of t he papers in th is field deal wi th crys ta l l iza t ion of po l ym er s f r o m mel t s a t high pressure [1-7]. I t was shown t h a t , a t cer ta in me l t t e m p e r a t u r e s and pressure, in bu lk samples of l inear po lye thy lene (PE) crys ta l l i tes f o r m wi th s t ra igh tened chains. A m u c h smaller n u m b e r of pape r s was concerned wi th effect of annea l ing under pressure a t t e m p e r a t u r e s lower t h a n the mel t ing po in t on po lymer s t ruc ture [8-11] and, in par t icu lar , on the s t ruc tu re of or iented samples [12-14]. Never theless , a de ta i led s t u d y of s t ruc tu ra l changes in or iented po lymer s dur ing annea l ing under pressure is of u n d o u b t e d in teres t f r o m the po in t of v iew of unde r s t and ing the m e c h a n i s m of crys ta l l iza t ion and recrysta l l izat ion. I n a r epo r t p rev ious ly publ i shed b y the au thor s [14] it was shown for the first t i m e t h a t hea t t r e a t m e n t of or iented P E under pressure unde r cer ta in condit ions is accompan ied b y the f o r m a t i o n of crys ta l l i tes wi th s t r a igh tened chains. This p a p e r seeks to exam i ne sys t ema t i ca l ly the effect of annea l ing a t diffe ren t t e m p e r a t u r e s and pressures on the s t ruc ture of or iented l inear P E specimens.

Determining the melting point of oriented polyethylene

For linear polyethylene films elongated by 1500%, initial films and films annealed at a pressure of 7 t/cm2 at 250–270° initial m. p. (Tmelt) (eliminating effects of overheating and recrystallization) and dimensions of crystallites were determined in the direction of the L002 chain. A linear relation was derived between Tmelt and L002 according to the Thomson equation. The equilibrium value of Tmelt = 143·6° and free surface energy of the end surface of crystallites σe = 104 erg/cm2 was determined.

Crystallization and annealing of crystalline polymers under high pressure

Abstract Crystallization from melts and annealing of crystalline polymers of different morphology (bulk polymerized, single-crystalline and oriented) under high pressure were examined.

The kinetics of changes in the crystallite size and long period in stretched polyethylene directly at the annealing temperature

Abstract Wide- and small-angle X-ray analysis at high temperatures has been used to study the kinetics of the longitudinal and lateral dimension changes of crystallites and of their long period during the annealing of orientated PE near the melting point. The annealing causes some of the crystallites to melt inside the fibrils and the crystallization of the partial melt at the annealing temperature on the lateral surfaces of the crystallites. There is a simultaneous lengthening of the macromolecular folds in the amorphous zones. The sample cooling is accompanied by an additional crystallization of the macromolecular segments entering the boundary surfaces of the crystallites which increase the longitudinal dimension of the latter.

Study of melting and crystallization of polyethylene at high pressure

Abstract A study was made by differential thermal analysis (DTA) of the effect of pressure up to 8000 kg/cm 2 on melting and crystallization of linear polyethylene (PE). It was shown that thermograms of melting and crystallization at pressures of over 4000 kg/cm 2 are characterized by the presence of two peaks. The existence of two peaks is a general phenomenon for isotropic and oriented specimens, single crystals and specimens containing crystals with straightened chains. An X-ray study shows that c -axial orientation of elongated specimens during annealing at a pressure of 7000 kg/cm 2 only disappears after the second endothermic peak, i.e. this peak corresponds to the final melting point. The presence of a low temperature peak points to the existence of an intermediate high pressure phase. A phase diagram was plotted under high pressure using DTA results, for isotropic and oriented PE samples up to 8000 kg/cm 2 . A study was made of the effect of the degree of elongation on melting PE under a pressure of 7000 kg/cm 2 .

Electron microscope investigation of processes of high pressure annealing of oriented polyethylene

An electron microscope study was made of structural changes occurring in oriented linear PE specimens annealed under pressure of 7 t/cm2, varying the time of annealing and temperature. It is shown that within a particular temperature interval high pressure annealing leads to the formation of crystallites with average dimensions of the order of 2000 A along the molecular chains; the maximum crystallite dimensions may be as large as 7000 A. The recrystallization process of PE during high pressure annealing is described in the light of the main tenets of the kinetic theory of the crystallization of polymers.

The x-ray structural analysis of orientated polyethylene films modified with a polyacrylonitrile graft

Abstract Low pressure polyethylene (LPPE) films were modified before and after orientational stretching by a graft copolymerization with acrylonitrile (AN). Small angle X-ray scattering (SAXS) has been used to study the structure present in the grafted films directly during isometric heating in the range 20–160°C. The polyacrylonitrile (PAN) formed discontinuous graft structures which prevents the relaxation of the orientated LPPE macromolecules. The high heat resistance of the grafts ensured the retention of orientation in the LPPE melt and the restoration of a fibrillar film structure during crystallization. The grafts do not hinder the production of the fibrillar structure during the grafted film stretching nor do they destroy it during the AN grafting onto orientated films.

Study of melting and recrystallization of polyethylene in solvents

A differential scanning calorimeter was used to investigate melting and recrystallization in cooled 0·5–50% PE solutions in xylene and decalin. It was shown that linear PE of not too high molecular weight (Mν×106) is subject to intensive recrystallization during scanning in quenched and rapidly cooled solutions. Recrystallization takes place at the highest rate and most completely during annealing these solutions at the temperature of maximum exothermic effect of recrystallization.