Compression testing

Extensional flow occurs when molecules in a material move towards (compression) or away (tensile) from each other. Two assumptions are undertaken during the deformation of a polymer network by extension. However, the same principle applies in compression. The first assumption is the affine deformation assumption, which implies that the macroscopic deformation of a polymer network directly translates to the deformation of the microscopic structure (Flory, 1953).

In other words, the dimensional changes occurring in the directions of the principal axes (x, y, z) will lead to a change in length within these axes, which is given by the extension/compression ratio (λ1x,¸λ2y,¸λ2z). The second assumption implies that the volume of the test sample remains constant during the deformation of the polymer network (Flory, 1953; Treloar, 1973). Based on the second assumption, the implication is that extension/compression in one direction would result in respective contraction/expansion in the transverse dimension, as shown in Equation 4.28. Adding Equation 4.28 to the equation for the work of deformation per unit volume gives Equation 4.29, which is summed in Equation 4.30.

λ1= λ and λ2= λ3= λ^−1/2 (4.28)

Ifσ is the force per unit cross sectional area of the unstrained test sample.

σ =F

A (4.31)

CHAPTER4. EXPERIMENTAL METHODS AND SAMPLE CHARACTERISATION

Assuming the shape of the sample is that of a cube of unit edge length (Treloar, 1973), then,

σ = RT νe(λ − 1

Here,λ is the ratio of the final height of sample to the initial height, known as the extension/

compression ratio, W is the work done per unit volume, R is the universal gas constant (8.314 x 106 cm³Pa mol⁻¹K⁻¹), T is the absolute temperature (293 K),νeis the elastically effective chain per unit volume,σ is the compressive stress and G is the compressive modulus.

Anton Paar MCR301 was used herein for the compression testing of composite hydrogels.

The normal force data obtained from the compression tests are transformed to stress values using Equation 4.31 (Oyen, 2014), while the gap displacement data are converted to the deform-ation factor (λ −_λ¹2) (Flory, 1953). Therefore, by plottingσ vs (λ −_λ¹2), according to Equation 4.34, the compressive modulus G can be obtained as the slope.

The British standard for the compression testing of rubber and thermoplastics exist (BSISO7743, 2017). This standard however does not take into consideration the changes in volume fractions upon swelling of rubbery material in suitable fluid. To account for the changes occurring within a swollen polymer network, The Flory’s theory of swollen rubber elasticity was adopted (Flory, 1953; Gutowska et al., 1994; Muniz and Geuskens, 2001). The volume ratios of the swollen and relaxed samples are incorporated into Equation 4.32 to give Equation 4.35. The functionνe

(mol/cm³), is the elastically effective chains of a polymer network and can be obtained from Equation 4.35.

σ = RT νe(φr/φs)²³φs(λ − 1

λ²) (4.35)

Here,φr is the volume fraction of the polymer in the relaxed state, that is, the hydrogel volume fraction remaining after the excess crosslinker had been leached.φsis the volume fraction of polymer at the equilibrium swollen state. The volumes of each component are converted from their weights using the density of the materials. In this thesis density of 1.6 g/cm³was used for sodium alginate, 1.5 g/cm³for CNF and 0.9982 g/cm³for water at 20 °C.

[ Chapter Five \

Cellulose Swelling and CNF Production Using Chemical Pre-treatments

5.1 Introduction

The macrostructure and microstructural orientation of cellulose and the associated complex-ities of cellulose swelling and dissolution were presented in Section 2.5 and 2.6 of Chapter 2.

The swelling of cellulose was discussed, and cellulose swelling agents were grouped as either intercrystalline or intracrystalline swelling agents. Intercrystalline swelling agents are defined as chemical agents that swell cellulose without changing the crystalline structure or derivatising cellulose. Intracrystalline swelling agents are able to swell cellulose, cause changes in the crys-talline structures of cellulose and derivatise cellulose (Klemm et al., 1998b). Efficient cellulose swelling is a prerequisite to reducing the degree of mechanical processing needed to produce good quality cellulose nanofibrils (Nechyporchuk et al., 2016). This reduces the cost of energy associated with CNF production, provided that the swelling process does not involve exotic chemical pre-treatments, using expensive chemicals (Bharimalla et al., 2015).

The most abundant and natural swelling agent for cellulose is water. However, the swell-ing degree is not sufficient to enable cellulose to be easily nanofibrillated. The mechanical processing of aqueous suspensions of cellulose into CNF requires some sort of mechanical pre-treatment and many passes through a high shear homogeniser. During high shear processing, the fibrous material blocks the interaction chamber, thereby hampering the fibrillation process.

Hence, the use of many pre-treatment methods (Section 3.2.3 of Chapter 3).

Lokhande, 1978 found that aqueous mixtures of heterocyclic amines (piperidine,

mor-CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

pholine) resulted in increased cellulose swelling compared to using the amines or water alone.

The study showed that aqueous mixtures of these swelling agents do not lead to the derivatisa-tion of cellulose. Hence, they are regarded as intercrystalline swelling agents. In a patent by Graveson and English, 2016, morpholine was one of the swelling agents used to facilitate the fibrillation of cellulose to produce cellulose nanofibrils. However, it was not made clear how the properties of the CNF produced by the morpholine pre-treatment, compare to the properties of CNF from other chemical pre-treatments. Further investigations on the properties of CNF derived from the morpholine swelling process and how these compare with the properties of the carboxymethylated and TEMPO-mediated oxidised CNFs have been conducted and a summary of results published (Onyianta et al., 2018a).

Swelling experiments using two types of cellulose, MCC and sieved knife-milled cellulose, in morpholine and piperidine are presented in this chapter. MCC was chosen because observable swelling in the crystalline region would imply even greater swelling for cellulose fibres within the amorphous regions. Sieved cellulose fibres have also been used to investigate swelling in both the crystalline regions and amorphous regions. It well known that the amorphous region of cellulose contributes to its degree of swelling in liquid media (Klemm et al., 1998b). This is mainly because of the presence of large pores and interstitial spaces arising from the less ordered packing of the anhydroglucopyranose repeat unit within the amorphous regions (Klemm et al., 1998b). Subsequently, the preparation methods, characterisation and properties of CNF derived from three pre-treatment methods are presented. The morpholine pre-treatment process is considered to be a chemical pre-treatment. The properties of the resulting CNF were compared with those of the more popular chemical pre-treatment methods described in the literature, carboxymethylation and TEMPO-mediated oxidation.