Investigation of the spatial inhomogeneity in poly(acrylamide) (PAAm) gels by static light scattering technique

Poliakrilamid (PAAm) jellerinde ışık saçınım tekniği ile yapısal inhomojenitenin nasıl değiştiği incelenmiştir. PAAm jellerinde başlangıç monomer konsantrasyonu sabit tutularak, sadece çapraz bağlayıcı konsantrasyonu değiştirilmiştir. Işık saçınım ölçüm sonuçlarından polimer jellerinin, polimer çözeltilerinden daima daha fazla ışığı saçtığı bulunmuştur. Jellerde gözlenen bu fazla saçınım intensitesi çapraz bağlayıcı konsantrasyonu arttıkça belirgin bir şekilde artmaktadır. Bu ölçülen fazla saçınım intensitesi  değeri yapısal inhomojenitenin ölçüm parametrisi olduğundan çapraz bağlayıcı konsantrasyonu artıkça jellerdeki inhomojenite derecesinin arttığı sonucuna varılmıştır. Işık saçınım intensitelerinden Debye Bueche teorisi yardımıyla inhomojen bölgelerin boyutları x ve konsantrasyon dalgalanmalarının kareleri ortalaması  değerleri hesaplanmıştır. Çalışmanın ikinci bölümünde amonyum persülfat – N,N,N',N'-tetrametilendiamin (TEMED) redoks başlatıcı sistemi ile PAAm jelleri sentezlenmiş ve bekleme süresi ile saçınım  intensitelerinin değişimi takip edilmiştir. PAAm jellerinin zamanla saçınım intensitelerinin azalarak daha homojen hale geldiği gözlenmiştir. Çalışmanın üçüncü bölümünde ise çapraz bağlayıcı konsantrasyonu  sabit tutularak farklı başlangıç konsantrasyonlarında hazırlanan PAAm jellerinde sentez sonrası ve şişme sonrası dengeye geldikten sonra yapısal inhomojenileri incelenmiş ve inhomojenite derecesinin maksimum olduğu kritik bir konsantrasyonun varlığı tespit edilmiştir. Elde edilen sonuçlar Debye Bueche teorisi yardımı ile açıklanarak nedenleri tartışılmıştır.  Anahtar kelimeler: Poliakrilamid, inhomojenite, ışık saçınımı.Hydrogels are important materials from both fundamental and technological points. Swelling properties and the elastic behavior of hydrogels have been intensively studied in the last four decades. However, theories are still unable to predict their physical properties from the synthesis conditions. This is due to the several non-idealities of the gel formation system such as the different and conversion-dependent reactivities of the vinyl groups, cyclization, multiple crosslinking, and diffusion-controlled reactions (Funke et al., 1998). In contrast to ideal gels with a homogeneous distribution of crosslinks, real gels always exhibit an inhomogeneous crosslink density distribution, known as the spatial gel inhomogeneity. The inhomogeneities present in gels are of considerable interest and importance in attempts to characterize such materials physically. From the practical point of view, spatial inhomogeneity is undesirable because it dramatically reduces the optical clarity and strength of gels, which are properties closely connected with many industrial applications such as contact lenses, super absorbents, cosmetics, etc. Since the spatial gel inhomogeneity necessarily produces local concentration fluctuations in gels, scattering methods have been used to investigate the inhomogeneities. The gel inhomogeneity can be manifested by comparing the scattering intensities from the gel and from a semi-dilute solution of the same polymer at the same concentration. The excess scattering over the scattering from polymer solution is related to the degree of the inhomogeneities in gels.  In general, the gel inhomogeneity increases with the gel crosslink density due to the simultaneous increase of the extent of network imperfections producing regions more or less rich in crosslinks  On the other hand, the inhomogeneity decreases with the ionization degree of gels due to the effects of the mobile counter ions, electrostatic repulsion and the Donnan potential. Degree of swelling of gels subjected to scattering measurements also affects the scattering intensities. Bueche reported in 1970 the enhancement of scattering intensity associated with the swelling of polymer Networks. The scattered light intensity from gels was found to be much larger than that of the polymer network alone. Later, various types of gels have been investigated at different swelling stages.The results show that the scattering intensity at low scattering vectors is enhanced as the swelling degree is increased. This behavior was interpreted as the enhancement of the difference of polymer concentration between the more and the less crosslinked regions. The spatial inhomogeneity in (PAAm) gels has been investigated with the static light scattering technique. In the first section, the gels were prepared using N,N'-methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed initial monomer concentration but at various crosslink densities. Elasticity measurements show that 80 to 90 % of the crosslinker molecules used in the hydrogel preparation were wasted by cyclization and multiple crosslinking reactions during the gel formation process. Light scattering measurements show that the excess scattering from gels increases with increasing BAAm content.  Since the excess scattering Rex(q) is a measure of the spatial inhomogeneity in a gel, this indicates that the PAAm gel becomes more inhomogeneous with increasing BAAm concentration. The size of the inhomogeneous regins in gels  as well as the mean square fluctuations in the reflactive index were calculated using the light scattering data of gels togetter with the Debye- bueche theory. In the second section ammonium persulfateN,N,N',N'-tetramethylethylenediamine redox initiator system was used to initiate the polymerization reactions and to follow up scattering intensities as a function of time of aging gels in the synthesis reactor. It was shown that the excess scattering decreases,  PAAm gel becomes increasingly homogeneous with increasing time of aging. The scattered light intensities from aged gels were interpreted using the Debye-Bueche analysis and a thermodynamic model was developed. In the third section the gels were prepared using N,N?-methylenebis(acrylamide) (BAAm) as a crosslinker at a fixed crosslinker ratio X but at various initial monomer concentrations. The gels, both at the state of preparation and at the equilibrium swollen state in water, exhibit a maximum degree of spatial gel inhomogeneity at a critical monomer concentration.  Keyword: Inhomogeneity, polyAcrylamide, light scattering