Effect of the Precise Branching of Polyethylene at Each 21st CH₂ Group on Its Phase Transitions, Crystal Structure, and Morphology

Abstract

Three linear polyethylenes with branches at every 21st backbone atom have been analyzed by differential scanning calorimetry (DSC) and quasi-isothermal, temperature-modulated DSC. The branches were methyl (PE1M), dimethyl (PE2M), and ethyl groups (PE1E). Linear polyethylene (HDPE) and atactic poly-(octadecyl acrylate) (PODA) were also analyzed. All were compared to a random poly(ethylene-co-octene-1) of similar branch concentration (LLDPE) and poly(4,4′-phthaloimidobenzoyldoeicosyleneoxycarbonyl) (PEIM-22). The HDPE has the highest melting temperature and crystallinity with relatively large contributions of reversing melting when grown as folded-chain crystals. The precisely branched polyethylenes and copolymers have lower melting temperatures and heats of fusion. Of the branched samples, PE1M crystallizes more readily, followed by PE1E and PE2M, with PE2M showing cold crystallization. In contrast to paraffins of equal length which melt fully reversibly, the precisely designed, branched polymers melt largely irreversibly with small amounts of reversing melting, which is least for the best-grown crystals. The PE1M forms monoclinic, PE1E, pseudohexagonal, or triclinic crystals, and PE2M has a multitude of crystal structures.

Crystallization and melting of a branched polyethylene with precisely controlled chemical structure

“…1 A single, low-temperature exotherm at 284.4 K characterizes the DSC on cooling. It is interesting that in the subsequent heating, PE2M also starts melting at very low temperature with a melting peak at 290.3 K, followed by an exotherm of cold-crystallization and crystal perfection (annealing) at 293.4 K. Assuming the crystallization and first melting peak refer to the same crystals, their supercooling is only 5.9 K. After crystal perfection, a second and third melting peak appear at 308.4 and 316.5 K, respectively.…”

“…The conclusions from the earlier research were as follows: 1 First, it was documented that the melting temperature of the crystallizing sequences is close to T m o of eicosane (309.8 K) and tetracontane (333 K), 80-100 K below the melting temperature of HDPE. This decrease is independent of crystallinity and type of crystal, and applies even to the broad peak of LLDPE of the same average length of crystallizable sequences.…”

“…1 These novel copolymers were synthesized via acyclic diene metathesis, ADMET 2 and compared to thermal analyses of polymers with similarly precise CH 2 -sequence lengths: poly(octadecyl acrylate) (PODA), 1 which has the CH 2 -sequences in the side chain, and poly(4,4 0 -phthaloimidobenzoyldoeicosyleneoxycarbonyl) , 3 which has the CH 2 -sequences in the main chain. Furthermore, the comparison was extended to a randomly branched, linear-low-density polyethylene of similar 1-octene content (LLDPE), 4,5 as well as a high-density polyethylene (HDPE) 1 and the paraffin n-C 20 H 42 (eico-sane) and n-C 40 H 82 (tetracontane). 6 This research into macromolecules with precise lengths of crystallizable units leads to insights into the nature of crystallization, melting, and decoupling of chainsequences within macromolecules.…”

“…7,8 The orthorhombic crystal structure of HDPE was not observed in these copolymers. 1 Instead, all samples have different, paraffin-like lateral packing, such as the frequently discussed hexagonal, monoclinic or triclinic polyethylene and paraffin structures. 9,10 Third, the crystals are thermodynamically decoupled from the amorphous segments, but transmit strains due to restrictions of volume and conformation, as documented by the broadening of the glass transition to higher temperature.…”

“…1). 1 The bottom curve in Figure 1 shows that on slow, stepwise cooling, as used in quasi-isothermal temperature-modulated DSC (TMDSC), the endotherms change in relative intensity, the sharp cold crystallization exotherm disappears, and the initial crystallinity increases somewhat. This behavior is studied in more detail in this paper to gain a better understanding of the properties of precisely-branched copolymers.…”