Structural transformations and water associate interactions in poly-N-vinylcaprolactam–water system

doi:10.1016/S0014-3057(98)00114-1

European Polymer Journal

Volume 35, Issue 2, February 1999, Pages 305-316

https://doi.org/10.1016/S0014-3057(98)00114-1 Get rights and content

Abstract

Structural transformations and water associate interactions were studied in poly-N-vinylcaprolactam (PVCL)–H₂O system by various methods (IR-spectroscopy, quantum-chemical calculations, DSC and light microscopy). A number of features of PVCL macromolecules were first found: (i) the conformation type of a side seven-member ring deduced by quantum-chemical calculations is the “chair”; (ii) the main chain of PVCL prepared by usual radical polymerization has syndiotactic structure; (iii) the calculation of charge distribution on a CO oxygen atom in the analogue of PVCL chainlink gives the large electron density value being −0.362. The addition of water to PVCL changes greatly glass transition temperature (T_g). The DSC method shows a lowering of T_g from 147° (dry polymer) to −17°÷−38° at N (number of water molecules per unit) being 2.6÷8.0. Cold crystallization and subsequent melting of ice-like units formed of water associates in the presence of PVCL macromolecules were displayed at N=8 and 10. Crystallization under cooling happens only at the increased N (N>10). Sharp decrease of melting heat of ice-like units (ΔH_mt) from 225 to 48 J/g (334 J/g of ice in bulk water) occurs as N diminishes from 20 up to 8. PVCL macromolecules in aqueous solution are the strongest modifier of water associate structures being affected by the polarization action of high polar amide CO groups and by specific configurational and conformational structures of the polymer.

Introduction

The elucidation of factors stipulating the origination of a number of specific physico-chemical properties of water-soluble poly-N-vinylamides such as poly-N-vinylpyrrolidone (PVP), poly-N-vinylcaprolactam (PVCL) and other in aqueous solutions, is of great scientific and practical interest. It is well-known[1], that PVP possesses great ability to form complexes with molecules of various structure, namely polymers of natural and synthetic origin, negatively charged organic molecules, inorganic ions and other. PVP finds wide application in medicine2, 3, 4.

PVCL has higher ability to form complexes than PVP and precipitates in a separate phase from aqueous solution in a range of physiological temperatures (32°–34° for PVCL against 170° for PVP), though each ring of PVCL chainlink differs from that of PVP only by two methylene groups5, 6. This property allows one to apply PVCL for microcapsulation of enzymes and living cells[7].

However, despite significant efforts of a large number of researchers, physico–chemical nature of manifestation of these properties in aqueous solutions remains unclear. Each of these properties is apparently caused by combined action of a number of factors associated both with macromolecule structure and the nature of hydrate surrounding nearby polymer chain. In fact, the relationship between the structure of polymer and specific hydration of macromolecule in water was found in the case of poly(ethylene glycol)8, 9and poly(1,3-dioxolane)[10].

The action of side substitute structure on water associate behaviour was revealed in a series of poly-N-vinylamides [PVP, PVCL, poly-N-vinyl-N-methylacetamide (PVMA) and poly-N-vinyl-N-methylformamide (PVMF)][11]. It was established by DSC method that the greatest distortion of the structure of ice-like units formed in the presence of a polymer occurs in PVCL solutions: the effect is revealed even in solutions at 200–300 water molecules per PVCL chainlink.

This peculiarity of PVCL behaviour in aqueous solutions promoted our further research of PVCL–H₂O system at higher polymer concentrations when high local concentration of polar amide groups, surrounding water associates, is created. Such a situation arises in channels of reverse-osmosis membrane selective layers made of aromatic polyamides, nearby surface and in cavities of protein molecules, in active sites of enzymes etc.

In the present work the attention is mainly paid to phase transitions (melting and crystallization) in PVCL–water system studied by DSC method. An estimation of interaction energy between water molecules in water associates surrounded by polar amide groups was performed by the methods of quantum chemistry and molecular mechanics. Conformation of side ring and configuration of main PVCL chain were also determined to understand a role of PVCL macromolecule structure on water associate behaviour.

Section snippets

Experimental section

N-vinylcaprolactam (VCL) was synthesized from caprolactam and acetylene[12]. The conformation type of a seven-member VCL ring was determined by quantum-chemical calculations (PM-3 under AMPAC-3 software) and a molecule mechanics method (PC Model under MMX software). The calculated differences of formation energy of various conformers testify that the most favourable conformation of VCL is the “chair”, whereas the “bath” conformation is energetically unfavourable (Fig. 1). Charge distribution on

Interaction of water with a caprolactam ring by IR-spectroscopy and quantum-chemical calculations

IR-spectra of PVCL in dry form (a film) and at the presence of various amounts of water molecules per chainlink (N) testify that the frequency of stretching vibrations of CO (ν_CO) is shifted greatly from ν=1640 cm⁻¹ at N=0 to 1610 cm⁻¹ at N=7. The shift of ν_CO and considerable widening (Δν_1/2) of the absorption band indicate hydrogen bond formation of CO with water molecules. In fact, the value of Δν_1/2 makes 22 cm⁻¹ at N=0 and 40 cm⁻¹ at N=7. It was established also that spectral

Acknowledgements

The authors are very grateful to Professor Yu. K. Godovsky who allowed them to use the DSC instrument in the Laboratory of Composite Polymer Systems (Karpov Institute of Physical Chemistry) and to Russian Foundation for Fundamental Research (projects 95-03-08216 and 96-15-97608), INTAS (project 93-2049) for financial support.

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Structural transformations and water associate interactions in poly-N-vinylcaprolactam–water system

Abstract

Introduction

Section snippets

Experimental section

Interaction of water with a caprolactam ring by IR-spectroscopy and quantum-chemical calculations

Acknowledgements

Prog. Polym. Sci.

Prog. Polym. Sci.

Enzym. Microbiol. Technol.

Polymer

Arzneim.-Forsch. (Drug Res.)

Khim.-Farm. Zh.

J. Appl. Polym. Sci.

Macromolecules

Russian J. Phys. Chem.